--- a/browser/base/content/contentSearchUI.js
+++ b/browser/base/content/contentSearchUI.js
@@ -60,20 +60,16 @@ function ContentSearchUIController(input
   this._hideSuggestions();
 
   this._getSearchEngines();
   this._getStrings();
 }
 
 ContentSearchUIController.prototype = {
 
-  // The timeout (ms) of the remote suggestions.  Corresponds to
-  // SearchSuggestionController.remoteTimeout.  Uses
-  // SearchSuggestionController's default timeout if falsey.
-  remoteTimeout: undefined,
   _oneOffButtons: [],
   // Setting up the one off buttons causes an uninterruptible reflow. If we
   // receive the list of engines while the newtab page is loading, this reflow
   // may regress performance - so we set this flag and only set up the buttons
   // if it's set when the suggestions table is actually opened.
   _pendingOneOffRefresh: undefined,
 
   get defaultEngine() {
@@ -711,17 +707,16 @@ ContentSearchUIController.prototype = {
   },
 
   _getSuggestions: function() {
     this._stickyInputValue = this.input.value;
     if (this.defaultEngine) {
       this._sendMsg("GetSuggestions", {
         engineName: this.defaultEngine.name,
         searchString: this.input.value,
-        remoteTimeout: this.remoteTimeout,
       });
     }
   },
 
   _clearSuggestionRows: function() {
     while (this._suggestionsList.firstElementChild) {
       this._suggestionsList.firstElementChild.remove();
     }

--- a/browser/base/content/test/general/browser_aboutHome.js
+++ b/browser/base/content/test/general/browser_aboutHome.js
@@ -311,19 +311,16 @@ add_task(function* () {
     // Add a test engine that provides suggestions and switch to it.
     let currEngine = Services.search.currentEngine;
     let engine = yield promiseNewEngine("searchSuggestionEngine.xml");
     let p = promiseContentSearchChange(browser, engine.name);
     Services.search.currentEngine = engine;
     yield p;
 
     yield ContentTask.spawn(browser, null, function* () {
-      // Avoid intermittent failures.
-      content.wrappedJSObject.gContentSearchController.remoteTimeout = 5000;
-
       // Type an X in the search input.
       let input = content.document.getElementById("searchText");
       input.focus();
     });
 
     yield BrowserTestUtils.synthesizeKey("x", {}, browser);
 
     yield ContentTask.spawn(browser, null, function* () {

--- a/browser/base/content/test/general/contentSearchUI.js
+++ b/browser/base/content/test/general/contentSearchUI.js
@@ -22,17 +22,16 @@ var messageHandlers = {
       new content.ContentSearchUIController(input, input.parentNode, "test", "test");
     content.addEventListener("ContentSearchService", function listener(aEvent) {
       if (aEvent.detail.type == "State" &&
           gController.defaultEngine.name == ENGINE_NAME) {
         content.removeEventListener("ContentSearchService", listener);
         ack("init");
       }
     });
-    gController.remoteTimeout = 5000;
   },
 
   key: function(arg) {
     let keyName = typeof(arg) == "string" ? arg : arg.key;
     content.synthesizeKey(keyName, arg.modifiers || {});
     let wait = arg.waitForSuggestions ? waitForSuggestions : cb => cb();
     wait(ack.bind(null, "key"));
   },

--- a/browser/base/content/test/newtab/browser_newtab_search.js
+++ b/browser/base/content/test/newtab/browser_newtab_search.js
@@ -120,21 +120,16 @@ add_task(function* () {
 
   // Add the engine that provides search suggestions and switch to it.
   let suggestionEngine = yield promiseNewSearchEngine(ENGINE_SUGGESTIONS);
   searchEventsPromise = promiseSearchEvents(["CurrentEngine"]);
   Services.search.currentEngine = suggestionEngine;
   yield searchEventsPromise;
   yield* checkCurrentEngine(ENGINE_SUGGESTIONS);
 
-  // Avoid intermittent failures.
-  yield ContentTask.spawn(gBrowser.selectedBrowser, {}, function* () {
-    content.gSearch._contentSearchController.remoteTimeout = 5000;
-  });
-
   // Type an X in the search input.  This is only a smoke test.  See
   // browser_searchSuggestionUI.js for comprehensive content search suggestion
   // UI tests.
   let suggestionsOpenPromise = new Promise(resolve => {
     mm.addMessageListener("test:newtab-suggestions-open", function onResponse(message) {
       mm.removeMessageListener("test:newtab-suggestions-open", onResponse);
       resolve();
     });

--- a/browser/modules/ContentSearch.jsm
+++ b/browser/modules/ContentSearch.jsm
@@ -36,17 +36,17 @@ const MAX_SUGGESTIONS = 6;
  *
  * Inbound messages have the following types:
  *
  *   AddFormHistoryEntry
  *     Adds an entry to the search form history.
  *     data: the entry, a string
  *   GetSuggestions
  *     Retrieves an array of search suggestions given a search string.
- *     data: { engineName, searchString, [remoteTimeout] }
+ *     data: { engineName, searchString }
  *   GetState
  *     Retrieves the current search engine state.
  *     data: null
  *   GetStrings
  *     Retrieves localized search UI strings.
  *     data: null
  *   ManageEngines
  *     Opens the search engine management window.
@@ -254,28 +254,27 @@ this.ContentSearch = {
       };
       win.openUILinkIn(submission.uri.spec, where, params);
     }
     win.BrowserSearch.recordSearchInTelemetry(engine, data.healthReportKey,
                                               { selection: data.selection });
     return;
   },
 
-  getSuggestions: Task.async(function* (engineName, searchString, browser, remoteTimeout = null) {
+  getSuggestions: Task.async(function* (engineName, searchString, browser) {
     let engine = Services.search.getEngineByName(engineName);
     if (!engine) {
       throw new Error("Unknown engine name: " + engineName);
     }
 
     let browserData = this._suggestionDataForBrowser(browser, true);
     let { controller } = browserData;
     let ok = SearchSuggestionController.engineOffersSuggestions(engine);
     controller.maxLocalResults = ok ? MAX_LOCAL_SUGGESTIONS : MAX_SUGGESTIONS;
     controller.maxRemoteResults = ok ? MAX_SUGGESTIONS : 0;
-    controller.remoteTimeout = remoteTimeout || undefined;
     let priv = PrivateBrowsingUtils.isBrowserPrivate(browser);
     // fetch() rejects its promise if there's a pending request, but since we
     // process our event queue serially, there's never a pending request.
     this._currentSuggestion = { controller: controller, target: browser };
     let suggestions = yield controller.fetch(searchString, priv, engine);
     this._currentSuggestion = null;
 
     // suggestions will be null if the request was cancelled

--- a/browser/modules/test/browser_ContentSearch.js
+++ b/browser/modules/test/browser_ContentSearch.js
@@ -1,20 +1,16 @@
 /* This Source Code Form is subject to the terms of the Mozilla Public
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
 const TEST_MSG = "ContentSearchTest";
 const CONTENT_SEARCH_MSG = "ContentSearch";
 const TEST_CONTENT_SCRIPT_BASENAME = "contentSearch.js";
 
-// This timeout is absurdly high to avoid random failures like bug 1087120.
-// That bug was reported when the timeout was 5 seconds, so let's try 10.
-const SUGGESTIONS_TIMEOUT = 10000;
-
 var gMsgMan;
 
 add_task(function* GetState() {
   yield addTab();
   gMsgMan.sendAsyncMessage(TEST_MSG, {
     type: "GetState",
   });
   let msg = yield waitForTestMsg("State");
@@ -187,17 +183,16 @@ add_task(function* GetSuggestions_AddFor
 
   // Send GetSuggestions using the test engine.  Its suggestions should appear
   // in the remote suggestions in the Suggestions response below.
   gMsgMan.sendAsyncMessage(TEST_MSG, {
     type: "GetSuggestions",
     data: {
       engineName: engine.name,
       searchString: searchStr,
-      remoteTimeout: SUGGESTIONS_TIMEOUT,
     },
   });
 
   // Check the Suggestions response.
   let msg = yield waitForTestMsg("Suggestions");
   checkMsg(msg, {
     type: "Suggestions",
     data: {
@@ -223,17 +218,16 @@ add_task(function* GetSuggestions_AddFor
   yield deferred.promise;
 
   // Send GetSuggestions again.
   gMsgMan.sendAsyncMessage(TEST_MSG, {
     type: "GetSuggestions",
     data: {
       engineName: engine.name,
       searchString: searchStr,
-      remoteTimeout: SUGGESTIONS_TIMEOUT,
     },
   });
 
   // The formHistory suggestions in the Suggestions response should be empty.
   msg = yield waitForTestMsg("Suggestions");
   checkMsg(msg, {
     type: "Suggestions",
     data: {

--- a/browser/modules/test/head.js
+++ b/browser/modules/test/head.js
@@ -62,20 +62,16 @@ function checkKeyedScalar(scalars, scala
  *        The browser that contains the content.
  * @param {String} text
  *        The string to write in the search field.
  * @param {String} fieldName
  *        The name of the field to write to.
  */
 let typeInSearchField = Task.async(function* (browser, text, fieldName) {
   yield ContentTask.spawn(browser, [fieldName, text], function* ([contentFieldName, contentText]) {
-    // Avoid intermittent failures.
-    if (contentFieldName === "searchText") {
-      content.wrappedJSObject.gContentSearchController.remoteTimeout = 5000;
-    }
     // Put the focus on the search box.
     let searchInput = content.document.getElementById(contentFieldName);
     searchInput.focus();
     searchInput.value = contentText;
   });
 });
 
 /**

new file mode 100644
--- /dev/null
+++ b/dom/base/OrderedTimeoutIterator.h
@@ -0,0 +1,187 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef mozilla_dom_OrderedTimeoutIterator_h__
+#define mozilla_dom_OrderedTimeoutIterator_h__
+
+#include "mozilla/RefPtr.h"
+#include "mozilla/dom/Timeout.h"
+#include "mozilla/dom/TimeoutManager.h"
+
+namespace mozilla {
+namespace dom {
+
+// This class implements and iterator which iterates the normal and tracking
+// timeouts lists simultaneously in the mWhen order.
+class MOZ_STACK_CLASS OrderedTimeoutIterator final {
+public:
+  typedef TimeoutManager::Timeouts Timeouts;
+  typedef Timeouts::TimeoutList    TimeoutList;
+
+  // Passing null for aNormalStopAt or aTrackingStopAt means that the
+  // corresponding timeout list should be iterated all the way to the end.
+  OrderedTimeoutIterator(Timeouts& aNormalTimeouts,
+                         Timeouts& aTrackingTimeouts,
+                         Timeout* aNormalStopAt,
+                         Timeout* aTrackingStopAt)
+    : mNormalTimeouts(aNormalTimeouts.mTimeoutList),
+      mTrackingTimeouts(aTrackingTimeouts.mTimeoutList),
+      mNormalIter(mNormalTimeouts.getFirst()),
+      mTrackingIter(mTrackingTimeouts.getFirst()),
+      mNormalStopAt(aNormalStopAt),
+      mTrackingStopAt(aTrackingStopAt),
+      mKind(Kind::None),
+      mUpdateIteratorCalled(true)
+  {
+  }
+
+  // Return the current timeout and move to the next one.
+  // Unless this is the first time calling Next(), you must call
+  // UpdateIterator() before calling this method.
+  Timeout* Next()
+  {
+    MOZ_ASSERT(mUpdateIteratorCalled);
+    MOZ_ASSERT_IF(mNormalIter && mNormalIter != mNormalStopAt,
+                  mNormalIter->isInList());
+    MOZ_ASSERT_IF(mTrackingIter && mTrackingIter != mTrackingStopAt,
+                  mTrackingIter->isInList());
+
+    mUpdateIteratorCalled = false;
+    mKind = Kind::None;
+    Timeout* timeout = nullptr;
+    if (mNormalIter == mNormalStopAt) {
+      if (mTrackingIter == mTrackingStopAt) {
+        // We have reached the end of both lists.  Bail out!
+        return nullptr;
+      } else {
+        // We have reached the end of the normal timeout list, select the next
+        // tracking timeout.
+        timeout = mTrackingIter;
+        mKind = Kind::Tracking;
+      }
+    } else if (mTrackingIter == mTrackingStopAt) {
+      // We have reached the end of the tracking timeout list, select the next
+      // normal timeout.
+      timeout = mNormalIter;
+      mKind = Kind::Normal;
+    } else {
+      // If we have a normal and a tracking timer, return the one with the
+      // smaller mWhen (and prefer the timeout with a lower ID in case they are
+      // equal.) Otherwise, return whichever iterator has an item left,
+      // preferring a non-tracking timeout again.  Note that in practice, even
+      // if a web page calls setTimeout() twice in a row, it should get
+      // different mWhen values, so in practice we shouldn't fall back to
+      // comparing timeout IDs.
+      if (mNormalIter && mTrackingIter &&
+          mNormalIter != mNormalStopAt &&
+          mTrackingIter != mTrackingStopAt &&
+          (mTrackingIter->mWhen < mNormalIter->mWhen ||
+           (mTrackingIter->mWhen == mNormalIter->mWhen &&
+            mTrackingIter->mTimeoutId < mNormalIter->mTimeoutId))) {
+        timeout = mTrackingIter;
+        mKind = Kind::Tracking;
+      } else if (mNormalIter && mNormalIter != mNormalStopAt) {
+        timeout = mNormalIter;
+        mKind = Kind::Normal;
+      } else if (mTrackingIter && mTrackingIter != mTrackingStopAt) {
+        timeout = mTrackingIter;
+        mKind = Kind::Tracking;
+      }
+    }
+    if (!timeout) {
+      // We didn't find any suitable iterator.  This can happen for example
+      // when getNext() in UpdateIterator() returns nullptr and then Next()
+      // gets called.  Bail out!
+      return nullptr;
+    }
+
+    MOZ_ASSERT(mKind != Kind::None);
+
+    // Record the current timeout we just found.
+    mCurrent = timeout;
+    MOZ_ASSERT(mCurrent);
+
+    return mCurrent;
+  }
+
+  // Prepare the iterator for the next call to Next().
+  // This method can be called as many times as needed.  Calling this more than
+  // once is helpful in cases where we expect the timeouts list has been
+  // modified before we got a chance to call Next().
+  void UpdateIterator()
+  {
+    MOZ_ASSERT(mKind != Kind::None);
+    // Update the winning iterator to point to the next element.  Also check to
+    // see if the other iterator is still valid, otherwise reset it to the
+    // beginning of the list.  This is needed in case a timeout handler removes
+    // the timeout pointed to from one of our iterators.
+    if (mKind == Kind::Normal) {
+      mNormalIter = mCurrent->getNext();
+      if (mTrackingIter && mTrackingIter != mTrackingStopAt &&
+          !mTrackingIter->isInList()) {
+        mTrackingIter = mTrackingTimeouts.getFirst();
+      }
+    } else {
+      mTrackingIter = mCurrent->getNext();
+      if (mNormalIter && mNormalIter != mNormalStopAt &&
+          !mNormalIter->isInList()) {
+        mNormalIter = mNormalTimeouts.getFirst();
+      }
+    }
+
+    mUpdateIteratorCalled = true;
+  }
+
+  // This function resets the iterator to a defunct state.  It should only be
+  // used when we want to forcefully sever all of the strong references this
+  // class holds.
+  void Clear()
+  {
+    // Release all strong references.
+    mNormalIter = nullptr;
+    mTrackingIter = nullptr;
+    mCurrent = nullptr;
+    mKind = Kind::None;
+    mUpdateIteratorCalled = true;
+  }
+
+  // Returns true if the previous call to Next() picked a normal timeout.
+  // Cannot be called before Next() has been called.  Note that the result of
+  // this method is only affected by Next() and not UpdateIterator(), so calling
+  // UpdateIterator() before calling this is allowed.
+  bool PickedNormalIter() const
+  {
+    MOZ_ASSERT(mKind != Kind::None);
+    return mKind == Kind::Normal;
+  }
+
+  // Returns true if the previous call to Next() picked a tracking timeout.
+  // Cannot be called before Next() has been called.  Note that the result of
+  // this method is only affected by Next() and not UpdateIterator(), so calling
+  // UpdateIterator() before calling this is allowed.
+  bool PickedTrackingIter() const
+  {
+    MOZ_ASSERT(mKind != Kind::None);
+    return mKind == Kind::Tracking;
+  }
+
+private:
+  TimeoutList& mNormalTimeouts;          // The list of normal timeouts.
+  TimeoutList& mTrackingTimeouts;        // The list of tracking timeouts.
+  RefPtr<Timeout> mNormalIter;           // The iterator over the normal timeout list.
+  RefPtr<Timeout> mTrackingIter;         // The iterator over the tracking timeout list.
+  void* mNormalStopAt;                   // Where to stop iterating the normal list at.
+  void* mTrackingStopAt;                 // Where to stop iterating the tracking list at.
+  RefPtr<Timeout> mCurrent;              // The current timeout that Next() just found.
+  enum class Kind { Normal, Tracking, None };
+  Kind mKind;                            // The kind of iterator picked the last time.
+  DebugOnly<bool> mUpdateIteratorCalled; // Whether we have called UpdateIterator() before calling Next().
+};
+
+}
+}
+
+#endif

--- a/dom/base/Timeout.cpp
+++ b/dom/base/Timeout.cpp
@@ -93,20 +93,20 @@ Timeout::InitTimer(nsIEventTarget* aTarg
     MOZ_ALWAYS_SUCCEEDS(mTimer->Cancel());
     MOZ_ALWAYS_SUCCEEDS(mTimer->SetTarget(aTarget));
   }
 
   return mTimer->InitWithNameableFuncCallback(
     TimerCallback, this, aDelay, nsITimer::TYPE_ONE_SHOT, TimerNameCallback);
 }
 
-// Return true if this timeout has a refcount of 1. This is used to check
+// Return true if this timeout has a refcount of aCount. This is used to check
 // that dummy_timeout doesn't leak from nsGlobalWindow::RunTimeout.
 #ifdef DEBUG
 bool
-Timeout::HasRefCntOne() const
+Timeout::HasRefCnt(uint32_t aCount) const
 {
-  return mRefCnt.get() == 1;
+  return mRefCnt.get() == aCount;
 }
 #endif // DEBUG
 
 } // namespace dom
 } // namespace mozilla

--- a/dom/base/Timeout.h
+++ b/dom/base/Timeout.h
@@ -40,17 +40,17 @@ public:
   // The target may be specified to use a particular event queue for the
   // resulting timer runnable.  A nullptr target will result in the
   // default main thread being used.
   nsresult InitTimer(nsIEventTarget* aTarget, uint32_t aDelay);
 
   enum class Reason { eTimeoutOrInterval, eIdleCallbackTimeout };
 
 #ifdef DEBUG
-  bool HasRefCntOne() const;
+  bool HasRefCnt(uint32_t aCount) const;
 #endif // DEBUG
 
   // Window for which this timeout fires
   RefPtr<nsGlobalWindow> mWindow;
 
   // The actual timer object
   nsCOMPtr<nsITimer> mTimer;
 

--- a/dom/base/TimeoutManager.cpp
+++ b/dom/base/TimeoutManager.cpp
@@ -5,16 +5,17 @@
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
 #include "TimeoutManager.h"
 #include "nsGlobalWindow.h"
 #include "mozilla/ThrottledEventQueue.h"
 #include "mozilla/TimeStamp.h"
 #include "nsITimeoutHandler.h"
 #include "mozilla/dom/TabGroup.h"
+#include "OrderedTimeoutIterator.h"
 
 using namespace mozilla;
 using namespace mozilla::dom;
 
 static int32_t              gRunningTimeoutDepth       = 0;
 
 // The default shortest interval/timeout we permit
 #define DEFAULT_MIN_TIMEOUT_VALUE 4 // 4ms
@@ -28,16 +29,23 @@ TimeoutManager::DOMMinTimeoutValue() con
   // Don't use the background timeout value when there are audio contexts
   // present, so that background audio can keep running smoothly. (bug 1181073)
   bool isBackground = !mWindow.AsInner()->HasAudioContexts() &&
     mWindow.IsBackgroundInternal();
   return
     std::max(isBackground ? gMinBackgroundTimeoutValue : gMinTimeoutValue, value);
 }
 
+#define TRACKING_SEPARATE_TIMEOUT_BUCKETING_STRATEGY 0 // Consider all timeouts coming from tracking scripts as tracking
+// These strategies are useful for testing.
+#define ALL_NORMAL_TIMEOUT_BUCKETING_STRATEGY        1 // Consider all timeouts as normal
+#define ALTERNATE_TIMEOUT_BUCKETING_STRATEGY         2 // Put every other timeout in the list of tracking timeouts
+#define RANDOM_TIMEOUT_BUCKETING_STRATEGY            3 // Put timeouts into either the normal or tracking timeouts list randomly
+static int32_t gTimeoutBucketingStrategy = 0;
+
 // The number of nested timeouts before we start clamping. HTML5 says 1, WebKit
 // uses 5.
 #define DOM_CLAMP_TIMEOUT_NESTING_LEVEL 5
 
 // The longest interval (as PRIntervalTime) we permit, or that our
 // timer code can handle, really. See DELAY_INTERVAL_LIMIT in
 // nsTimerImpl.h for details.
 #define DOM_MAX_TIMEOUT_VALUE    DELAY_INTERVAL_LIMIT
@@ -107,16 +115,19 @@ void
 TimeoutManager::Initialize()
 {
   Preferences::AddIntVarCache(&gMinTimeoutValue,
                               "dom.min_timeout_value",
                               DEFAULT_MIN_TIMEOUT_VALUE);
   Preferences::AddIntVarCache(&gMinBackgroundTimeoutValue,
                               "dom.min_background_timeout_value",
                               DEFAULT_MIN_BACKGROUND_TIMEOUT_VALUE);
+  Preferences::AddIntVarCache(&gTimeoutBucketingStrategy,
+                              "dom.timeout_bucketing_strategy",
+                              TRACKING_SEPARATE_TIMEOUT_BUCKETING_STRATEGY);
 }
 
 uint32_t
 TimeoutManager::GetTimeoutId(Timeout::Reason aReason)
 {
   switch (aReason) {
     case Timeout::Reason::eIdleCallbackTimeout:
       return ++mIdleCallbackTimeoutCounter;
@@ -128,17 +139,18 @@ TimeoutManager::GetTimeoutId(Timeout::Re
 
 nsresult
 TimeoutManager::SetTimeout(nsITimeoutHandler* aHandler,
                            int32_t interval, bool aIsInterval,
                            Timeout::Reason aReason, int32_t* aReturn)
 {
   // If we don't have a document (we could have been unloaded since
   // the call to setTimeout was made), do nothing.
-  if (!mWindow.GetExtantDoc()) {
+  nsCOMPtr<nsIDocument> doc = mWindow.GetExtantDoc();
+  if (!doc) {
     return NS_OK;
   }
 
   // Disallow negative intervals.  If aIsInterval also disallow 0,
   // because we use that as a "don't repeat" flag.
   interval = std::max(aIsInterval ? 1 : 0, interval);
 
   // Make sure we don't proceed with an interval larger than our timer
@@ -223,31 +235,57 @@ TimeoutManager::SetTimeout(nsITimeoutHan
     // This is checking |interval|, not realInterval, on purpose,
     // because our lower bound for |realInterval| could be pretty high
     // in some cases.
     if (interval <= delay) {
       timeout->mPopupState = mWindow.GetPopupControlState();
     }
   }
 
-  mTimeouts.Insert(timeout, mWindow.IsFrozen() ? Timeouts::SortBy::TimeRemaining
-                                               : Timeouts::SortBy::TimeWhen);
+  bool isTracking = false;
+  switch (gTimeoutBucketingStrategy) {
+  default:
+  case TRACKING_SEPARATE_TIMEOUT_BUCKETING_STRATEGY: {
+    const char* filename = nullptr;
+    uint32_t dummyLine = 0, dummyColumn = 0;
+    aHandler->GetLocation(&filename, &dummyLine, &dummyColumn);
+    isTracking = doc->IsScriptTracking(nsDependentCString(filename));
+    break;
+  }
+  case ALL_NORMAL_TIMEOUT_BUCKETING_STRATEGY:
+    // isTracking is already false!
+    break;
+  case ALTERNATE_TIMEOUT_BUCKETING_STRATEGY:
+    isTracking = (mTimeoutIdCounter % 2) == 0;
+    break;
+  case RANDOM_TIMEOUT_BUCKETING_STRATEGY:
+    isTracking = (rand() % 2) == 0;
+    break;
+  }
+
+  Timeouts::SortBy sort(mWindow.IsFrozen() ? Timeouts::SortBy::TimeRemaining
+                                           : Timeouts::SortBy::TimeWhen);
+  if (isTracking) {
+    mTrackingTimeouts.Insert(timeout, sort);
+  } else {
+    mNormalTimeouts.Insert(timeout, sort);
+  }
 
   timeout->mTimeoutId = GetTimeoutId(aReason);
   *aReturn = timeout->mTimeoutId;
 
   return NS_OK;
 }
 
 void
 TimeoutManager::ClearTimeout(int32_t aTimerId, Timeout::Reason aReason)
 {
   uint32_t timerId = (uint32_t)aTimerId;
 
-  ForEachTimeoutAbortable([&](Timeout* aTimeout) {
+  ForEachUnorderedTimeoutAbortable([&](Timeout* aTimeout) {
     if (aTimeout->mTimeoutId == timerId && aTimeout->mReason == aReason) {
       if (aTimeout->mRunning) {
         /* We're running from inside the aTimeout. Mark this
            aTimeout for deferred deletion by the code in
            RunTimeout() */
         aTimeout->mIsInterval = false;
       }
       else {
@@ -271,19 +309,21 @@ void
 TimeoutManager::RunTimeout(Timeout* aTimeout)
 {
   if (mWindow.IsSuspended()) {
     return;
   }
 
   NS_ASSERTION(!mWindow.IsFrozen(), "Timeout running on a window in the bfcache!");
 
-  Timeout* nextTimeout;
-  Timeout* last_expired_timeout;
-  Timeout* last_insertion_point;
+  Timeout* last_expired_normal_timeout = nullptr;
+  Timeout* last_expired_tracking_timeout = nullptr;
+  bool     last_expired_timeout_is_normal = false;
+  Timeout* last_normal_insertion_point = nullptr;
+  Timeout* last_tracking_insertion_point = nullptr;
   uint32_t firingDepth = mTimeoutFiringDepth + 1;
 
   // Make sure that the window and the script context don't go away as
   // a result of running timeouts
   nsCOMPtr<nsIScriptGlobalObject> windowKungFuDeathGrip(&mWindow);
   // Silence the static analysis error about windowKungFuDeathGrip.  Accessing
   // members of mWindow here is safe, because the lifetime of TimeoutManager is
   // the same as the lifetime of the containing nsGlobalWindow.
@@ -307,140 +347,233 @@ TimeoutManager::RunTimeout(Timeout* aTim
   }
 
   // The timeout list is kept in deadline order. Discover the latest timeout
   // whose deadline has expired. On some platforms, native timeout events fire
   // "early", but we handled that above by setting deadline to aTimeout->mWhen
   // if the timer fired early.  So we can stop walking if we get to timeouts
   // whose mWhen is greater than deadline, since once that happens we know
   // nothing past that point is expired.
-  last_expired_timeout = nullptr;
-  for (Timeout* timeout = mTimeouts.GetFirst();
-       timeout && timeout->mWhen <= deadline;
-       timeout = timeout->getNext()) {
-    if (timeout->mFiringDepth == 0) {
-      // Mark any timeouts that are on the list to be fired with the
-      // firing depth so that we can reentrantly run timeouts
-      timeout->mFiringDepth = firingDepth;
-      last_expired_timeout = timeout;
-
-      // Run available timers until we see our target timer.  After
-      // that, however, stop coalescing timers so we can yield the
-      // main thread.  Further timers that are ready will get picked
-      // up by their own nsITimer runnables when they execute.
-      //
-      // For chrome windows, however, we do coalesce all timers and
-      // do not yield the main thread.  This is partly because we
-      // trust chrome windows not to misbehave and partly because a
-      // number of browser chrome tests have races that depend on this
-      // coalescing.
-      if (timeout == aTimeout && !mWindow.IsChromeWindow()) {
+  {
+    // Use a nested scope in order to make sure the strong references held by
+    // the iterator are freed after the loop.
+    OrderedTimeoutIterator expiredIter(mNormalTimeouts,
+                                       mTrackingTimeouts,
+                                       nullptr,
+                                       nullptr);
+    while (true) {
+      Timeout* timeout = expiredIter.Next();
+      if (!timeout || timeout->mWhen > deadline) {
         break;
       }
+
+      if (timeout->mFiringDepth == 0) {
+        // Mark any timeouts that are on the list to be fired with the
+        // firing depth so that we can reentrantly run timeouts
+        timeout->mFiringDepth = firingDepth;
+        last_expired_timeout_is_normal = expiredIter.PickedNormalIter();
+        if (last_expired_timeout_is_normal) {
+          last_expired_normal_timeout = timeout;
+        } else {
+          last_expired_tracking_timeout = timeout;
+        }
+
+        // Run available timers until we see our target timer.  After
+        // that, however, stop coalescing timers so we can yield the
+        // main thread.  Further timers that are ready will get picked
+        // up by their own nsITimer runnables when they execute.
+        //
+        // For chrome windows, however, we do coalesce all timers and
+        // do not yield the main thread.  This is partly because we
+        // trust chrome windows not to misbehave and partly because a
+        // number of browser chrome tests have races that depend on this
+        // coalescing.
+        if (timeout == aTimeout && !mWindow.IsChromeWindow()) {
+          break;
+        }
+      }
+
+      expiredIter.UpdateIterator();
     }
   }
 
   // Maybe the timeout that the event was fired for has been deleted
   // and there are no others timeouts with deadlines that make them
   // eligible for execution yet. Go away.
-  if (!last_expired_timeout) {
+  if (!last_expired_normal_timeout && !last_expired_tracking_timeout) {
     return;
   }
 
   // Insert a dummy timeout into the list of timeouts between the
   // portion of the list that we are about to process now and those
   // timeouts that will be processed in a future call to
   // win_run_timeout(). This dummy timeout serves as the head of the
   // list for any timeouts inserted as a result of running a timeout.
-  RefPtr<Timeout> dummy_timeout = new Timeout();
-  dummy_timeout->mFiringDepth = firingDepth;
-  dummy_timeout->mWhen = now;
-  last_expired_timeout->setNext(dummy_timeout);
-  RefPtr<Timeout> timeoutExtraRef(dummy_timeout);
+  RefPtr<Timeout> dummy_normal_timeout = new Timeout();
+  dummy_normal_timeout->mFiringDepth = firingDepth;
+  dummy_normal_timeout->mWhen = now;
+  if (last_expired_timeout_is_normal) {
+    last_expired_normal_timeout->setNext(dummy_normal_timeout);
+  }
+
+  RefPtr<Timeout> dummy_tracking_timeout = new Timeout();
+  dummy_tracking_timeout->mFiringDepth = firingDepth;
+  dummy_tracking_timeout->mWhen = now;
+  if (!last_expired_timeout_is_normal) {
+    last_expired_tracking_timeout->setNext(dummy_tracking_timeout);
+  }
+
+  RefPtr<Timeout> timeoutExtraRef1(dummy_normal_timeout);
+  RefPtr<Timeout> timeoutExtraRef2(dummy_tracking_timeout);
 
-  last_insertion_point = mTimeouts.InsertionPoint();
-  // If we ever start setting insertion point to a non-dummy timeout, the logic
-  // in ResetTimersForThrottleReduction will need to change.
-  mTimeouts.SetInsertionPoint(dummy_timeout);
+  // Now we need to search the normal and tracking timer list at the same
+  // time to run the timers in the scheduled order.
 
-  for (Timeout* timeout = mTimeouts.GetFirst();
-       timeout != dummy_timeout && !mWindow.IsFrozen();
-       timeout = nextTimeout) {
-    nextTimeout = timeout->getNext();
+  last_normal_insertion_point = mNormalTimeouts.InsertionPoint();
+  if (last_expired_timeout_is_normal) {
+    // If we ever start setting insertion point to a non-dummy timeout, the logic
+    // in ResetTimersForThrottleReduction will need to change.
+    mNormalTimeouts.SetInsertionPoint(dummy_normal_timeout);
+  }
+
+  last_tracking_insertion_point = mTrackingTimeouts.InsertionPoint();
+  if (!last_expired_timeout_is_normal) {
+    // If we ever start setting mTrackingTimeoutInsertionPoint to a non-dummy timeout,
+    // the logic in ResetTimersForThrottleReduction will need to change.
+    mTrackingTimeouts.SetInsertionPoint(dummy_tracking_timeout);
+  }
 
-    if (timeout->mFiringDepth != firingDepth) {
-      // We skip the timeout since it's on the list to run at another
-      // depth.
-
-      continue;
-    }
+  // We stop iterating each list when we go past the last expired timeout from
+  // that list that we have observed above.  That timeout will either be the
+  // dummy timeout for the list that the last expired timeout came from, or it
+  // will be the next item after the last timeout we looked at (or nullptr if
+  // we have exhausted the entire list while looking for the last expired
+  // timeout).
+  {
+    // Use a nested scope in order to make sure the strong references held by
+    // the iterator are freed after the loop.
+    OrderedTimeoutIterator runIter(mNormalTimeouts,
+                                   mTrackingTimeouts,
+                                   last_expired_normal_timeout ?
+                                     last_expired_normal_timeout->getNext() :
+                                     nullptr,
+                                   last_expired_tracking_timeout ?
+                                     last_expired_tracking_timeout->getNext() :
+                                     nullptr);
+    while (!mWindow.IsFrozen()) {
+      Timeout* timeout = runIter.Next();
+      MOZ_ASSERT(timeout != dummy_normal_timeout &&
+                 timeout != dummy_tracking_timeout,
+                 "We should have stopped iterating before getting to the dummy timeout");
+      if (!timeout) {
+        // We have run out of timeouts!
+        break;
+      }
+      runIter.UpdateIterator();
 
-    if (mWindow.IsSuspended()) {
-      // Some timer did suspend us. Make sure the
-      // rest of the timers get executed later.
-      timeout->mFiringDepth = 0;
-      continue;
-    }
-
-    // The timeout is on the list to run at this depth, go ahead and
-    // process it.
-
-    // Get the script context (a strong ref to prevent it going away)
-    // for this timeout and ensure the script language is enabled.
-    nsCOMPtr<nsIScriptContext> scx = mWindow.GetContextInternal();
+      if (timeout->mFiringDepth != firingDepth) {
+        // We skip the timeout since it's on the list to run at another
+        // depth.
+        continue;
+      }
 
-    if (!scx) {
-      // No context means this window was closed or never properly
-      // initialized for this language.
-      continue;
-    }
+      if (mWindow.IsSuspended()) {
+        // Some timer did suspend us. Make sure the
+        // rest of the timers get executed later.
+        timeout->mFiringDepth = 0;
+        continue;
+      }
+
+      // The timeout is on the list to run at this depth, go ahead and
+      // process it.
 
-    // This timeout is good to run
-    bool timeout_was_cleared = mWindow.RunTimeoutHandler(timeout, scx);
+      // Get the script context (a strong ref to prevent it going away)
+      // for this timeout and ensure the script language is enabled.
+      nsCOMPtr<nsIScriptContext> scx = mWindow.GetContextInternal();
+
+      if (!scx) {
+        // No context means this window was closed or never properly
+        // initialized for this language.
+        continue;
+      }
+
+      // This timeout is good to run
+      bool timeout_was_cleared = mWindow.RunTimeoutHandler(timeout, scx);
+
+      if (timeout_was_cleared) {
+        // Make sure the iterator isn't holding any Timeout objects alive.
+        runIter.Clear();
 
-    if (timeout_was_cleared) {
-      // The running timeout's window was cleared, this means that
-      // ClearAllTimeouts() was called from a *nested* call, possibly
-      // through a timeout that fired while a modal (to this window)
-      // dialog was open or through other non-obvious paths.
-      MOZ_ASSERT(dummy_timeout->HasRefCntOne(), "dummy_timeout may leak");
-      Unused << timeoutExtraRef.forget().take();
+        // The running timeout's window was cleared, this means that
+        // ClearAllTimeouts() was called from a *nested* call, possibly
+        // through a timeout that fired while a modal (to this window)
+        // dialog was open or through other non-obvious paths.
+        // Note that if the last expired timeout corresponding to each list
+        // is null, then we should expect a refcount of two, since the
+        // dummy timeout for this queue was never injected into it, and the
+        // corresponding timeoutExtraRef variable hasn't been cleared yet.
+        if (last_expired_timeout_is_normal) {
+          MOZ_ASSERT(dummy_normal_timeout->HasRefCnt(1), "dummy_normal_timeout may leak");
+          MOZ_ASSERT(dummy_tracking_timeout->HasRefCnt(2), "dummy_tracking_timeout may leak");
+          Unused << timeoutExtraRef1.forget().take();
+        } else {
+          MOZ_ASSERT(dummy_normal_timeout->HasRefCnt(2), "dummy_normal_timeout may leak");
+          MOZ_ASSERT(dummy_tracking_timeout->HasRefCnt(1), "dummy_tracking_timeout may leak");
+          Unused << timeoutExtraRef2.forget().take();
+        }
+
+        mNormalTimeouts.SetInsertionPoint(last_normal_insertion_point);
+        mTrackingTimeouts.SetInsertionPoint(last_tracking_insertion_point);
+
+        return;
+      }
 
-      mTimeouts.SetInsertionPoint(last_insertion_point);
+      // If we have a regular interval timer, we re-schedule the
+      // timeout, accounting for clock drift.
+      bool needsReinsertion = RescheduleTimeout(timeout, now, !aTimeout);
+
+      // Running a timeout can cause another timeout to be deleted, so
+      // we need to reset the pointer to the following timeout.
+      runIter.UpdateIterator();
+
+      timeout->remove();
 
-      return;
+      if (needsReinsertion) {
+        // Insert interval timeout onto the corresponding list sorted in
+        // deadline order. AddRefs timeout.
+        if (runIter.PickedTrackingIter()) {
+          mTrackingTimeouts.Insert(timeout,
+                                   mWindow.IsFrozen() ? Timeouts::SortBy::TimeRemaining
+                                                      : Timeouts::SortBy::TimeWhen);
+        } else {
+          mNormalTimeouts.Insert(timeout,
+                                 mWindow.IsFrozen() ? Timeouts::SortBy::TimeRemaining
+                                                    : Timeouts::SortBy::TimeWhen);
+        }
+      }
+
+      // Release the timeout struct since it's possibly out of the list
+      timeout->Release();
     }
-
-    // If we have a regular interval timer, we re-schedule the
-    // timeout, accounting for clock drift.
-    bool needsReinsertion = RescheduleTimeout(timeout, now, !aTimeout);
-
-    // Running a timeout can cause another timeout to be deleted, so
-    // we need to reset the pointer to the following timeout.
-    nextTimeout = timeout->getNext();
-
-    timeout->remove();
-
-    if (needsReinsertion) {
-      // Insert interval timeout onto list sorted in deadline order.
-      // AddRefs timeout.
-      mTimeouts.Insert(timeout, mWindow.IsFrozen() ? Timeouts::SortBy::TimeRemaining
-                                                   : Timeouts::SortBy::TimeWhen);
-    }
-
-    // Release the timeout struct since it's possibly out of the list
-    timeout->Release();
   }
 
   // Take the dummy timeout off the head of the list
-  dummy_timeout->remove();
-  timeoutExtraRef = nullptr;
-  MOZ_ASSERT(dummy_timeout->HasRefCntOne(), "dummy_timeout may leak");
+  if (dummy_normal_timeout->isInList()) {
+    dummy_normal_timeout->remove();
+  }
+  timeoutExtraRef1 = nullptr;
+  MOZ_ASSERT(dummy_normal_timeout->HasRefCnt(1), "dummy_normal_timeout may leak");
+  if (dummy_tracking_timeout->isInList()) {
+    dummy_tracking_timeout->remove();
+  }
+  timeoutExtraRef2 = nullptr;
+  MOZ_ASSERT(dummy_tracking_timeout->HasRefCnt(1), "dummy_tracking_timeout may leak");
 
-  mTimeouts.SetInsertionPoint(last_insertion_point);
+  mNormalTimeouts.SetInsertionPoint(last_normal_insertion_point);
+  mTrackingTimeouts.SetInsertionPoint(last_tracking_insertion_point);
 
   MaybeApplyBackPressure();
 }
 
 void
 TimeoutManager::MaybeApplyBackPressure()
 {
   MOZ_ASSERT(NS_IsMainThread());
@@ -627,23 +760,32 @@ nsresult
 TimeoutManager::ResetTimersForThrottleReduction(int32_t aPreviousThrottleDelayMS)
 {
   MOZ_ASSERT(aPreviousThrottleDelayMS > 0);
 
   if (mWindow.IsFrozen() || mWindow.IsSuspended()) {
     return NS_OK;
   }
 
+  auto minTimeout = DOMMinTimeoutValue();
   Timeouts::SortBy sortBy = mWindow.IsFrozen() ? Timeouts::SortBy::TimeRemaining
                                                : Timeouts::SortBy::TimeWhen;
 
-  return mTimeouts.ResetTimersForThrottleReduction(aPreviousThrottleDelayMS,
-                                                   DOMMinTimeoutValue(),
-                                                   sortBy,
-                                                   mWindow.GetThrottledEventQueue());
+  nsresult rv = mNormalTimeouts.ResetTimersForThrottleReduction(aPreviousThrottleDelayMS,
+                                                                minTimeout,
+                                                                sortBy,
+                                                                mWindow.GetThrottledEventQueue());
+  NS_ENSURE_SUCCESS(rv, rv);
+  rv = mTrackingTimeouts.ResetTimersForThrottleReduction(aPreviousThrottleDelayMS,
+                                                         minTimeout,
+                                                         sortBy,
+                                                         mWindow.GetThrottledEventQueue());
+  NS_ENSURE_SUCCESS(rv, rv);
+
+  return NS_OK;
 }
 
 nsresult
 TimeoutManager::Timeouts::ResetTimersForThrottleReduction(int32_t aPreviousThrottleDelayMS,
                                                           int32_t aMinTimeoutValueMS,
                                                           SortBy aSortBy,
                                                           ThrottledEventQueue* aQueue)
 {
@@ -731,17 +873,17 @@ TimeoutManager::Timeouts::ResetTimersFor
   return NS_OK;
 }
 
 void
 TimeoutManager::ClearAllTimeouts()
 {
   bool seenRunningTimeout = false;
 
-  ForEachTimeout([&](Timeout* aTimeout) {
+  ForEachUnorderedTimeout([&](Timeout* aTimeout) {
     /* If RunTimeout() is higher up on the stack for this
        window, e.g. as a result of document.write from a timeout,
        then we need to reset the list insertion point for
        newly-created timeouts in case the user adds a timeout,
        before we pop the stack back to RunTimeout. */
     if (mRunningTimeout == aTimeout) {
       seenRunningTimeout = true;
     }
@@ -759,21 +901,23 @@ TimeoutManager::ClearAllTimeouts()
     // cleared and taken out of the list of timeouts
     aTimeout->mCleared = true;
 
     // Drop the count since we're removing it from the list.
     aTimeout->Release();
   });
 
   if (seenRunningTimeout) {
-    mTimeouts.SetInsertionPoint(nullptr);
+    mNormalTimeouts.SetInsertionPoint(nullptr);
+    mTrackingTimeouts.SetInsertionPoint(nullptr);
   }
 
   // Clear out our list
-  mTimeouts.Clear();
+  mNormalTimeouts.Clear();
+  mTrackingTimeouts.Clear();
 }
 
 void
 TimeoutManager::Timeouts::Insert(Timeout* aTimeout, SortBy aSortBy)
 {
   // Start at mLastTimeout and go backwards.  Don't go further than insertion
   // point, though.  This optimizes for the common case of insertion at the end.
   Timeout* prevSibling;
@@ -821,27 +965,27 @@ TimeoutManager::EndRunningTimeout(Timeou
   --gRunningTimeoutDepth;
 
   mRunningTimeout = aTimeout;
 }
 
 void
 TimeoutManager::UnmarkGrayTimers()
 {
-  ForEachTimeout([](Timeout* aTimeout) {
+  ForEachUnorderedTimeout([](Timeout* aTimeout) {
     if (aTimeout->mScriptHandler) {
       aTimeout->mScriptHandler->MarkForCC();
     }
   });
 }
 
 void
 TimeoutManager::Suspend()
 {
-  ForEachTimeout([](Timeout* aTimeout) {
+  ForEachUnorderedTimeout([](Timeout* aTimeout) {
     // Leave the timers with the current time remaining.  This will
     // cause the timers to potentially fire when the window is
     // Resume()'d.  Time effectively passes while suspended.
 
     // Drop the XPCOM timer; we'll reschedule when restoring the state.
     if (aTimeout->mTimer) {
       aTimeout->mTimer->Cancel();
       aTimeout->mTimer = nullptr;
@@ -854,17 +998,17 @@ TimeoutManager::Suspend()
 }
 
 void
 TimeoutManager::Resume()
 {
   TimeStamp now = TimeStamp::Now();
   DebugOnly<bool> _seenDummyTimeout = false;
 
-  ForEachTimeout([&](Timeout* aTimeout) {
+  ForEachUnorderedTimeout([&](Timeout* aTimeout) {
     // There's a chance we're being called with RunTimeout on the stack in which
     // case we have a dummy timeout in the list that *must not* be resumed. It
     // can be identified by a null mWindow.
     if (!aTimeout->mWindow) {
       NS_ASSERTION(!_seenDummyTimeout, "More than one dummy timeout?!");
       _seenDummyTimeout = true;
       return;
     }
@@ -900,17 +1044,17 @@ TimeoutManager::Resume()
     aTimeout->AddRef();
   });
 }
 
 void
 TimeoutManager::Freeze()
 {
   TimeStamp now = TimeStamp::Now();
-  ForEachTimeout([&](Timeout* aTimeout) {
+  ForEachUnorderedTimeout([&](Timeout* aTimeout) {
     // Save the current remaining time for this timeout.  We will
     // re-apply it when the window is Thaw()'d.  This effectively
     // shifts timers to the right as if time does not pass while
     // the window is frozen.
     if (aTimeout->mWhen > now) {
       aTimeout->mTimeRemaining = aTimeout->mWhen - now;
     } else {
       aTimeout->mTimeRemaining = TimeDuration(0);
@@ -923,24 +1067,32 @@ TimeoutManager::Freeze()
 }
 
 void
 TimeoutManager::Thaw()
 {
   TimeStamp now = TimeStamp::Now();
   DebugOnly<bool> _seenDummyTimeout = false;
 
-  ForEachTimeout([&](Timeout* aTimeout) {
+  ForEachUnorderedTimeout([&](Timeout* aTimeout) {
     // There's a chance we're being called with RunTimeout on the stack in which
     // case we have a dummy timeout in the list that *must not* be resumed. It
     // can be identified by a null mWindow.
     if (!aTimeout->mWindow) {
       NS_ASSERTION(!_seenDummyTimeout, "More than one dummy timeout?!");
       _seenDummyTimeout = true;
       return;
     }
 
     // Set mWhen back to the time when the timer is supposed to fire.
     aTimeout->mWhen = now + aTimeout->mTimeRemaining;
 
     MOZ_ASSERT(!aTimeout->mTimer);
   });
 }
+
+bool
+TimeoutManager::IsTimeoutTracking(uint32_t aTimeoutId)
+{
+  return mTrackingTimeouts.ForEachAbortable([&](Timeout* aTimeout) {
+      return aTimeout->mTimeoutId == aTimeoutId;
+    });
+}

--- a/dom/base/TimeoutManager.h
+++ b/dom/base/TimeoutManager.h
@@ -13,30 +13,36 @@ class nsITimeoutHandler;
 class nsGlobalWindow;
 
 namespace mozilla {
 
 class ThrottledEventQueue;
 
 namespace dom {
 
+class OrderedTimeoutIterator;
+
 // This class manages the timeouts in a Window's setTimeout/setInterval pool.
 class TimeoutManager final
 {
 public:
   explicit TimeoutManager(nsGlobalWindow& aWindow);
   TimeoutManager(const TimeoutManager& rhs) = delete;
   void operator=(const TimeoutManager& rhs) = delete;
 
   bool IsRunningTimeout() const { return mTimeoutFiringDepth > 0; }
 
   static uint32_t GetNestingLevel() { return sNestingLevel; }
   static void SetNestingLevel(uint32_t aLevel) { sNestingLevel = aLevel; }
 
-  bool HasTimeouts() const { return !mTimeouts.IsEmpty(); }
+  bool HasTimeouts() const
+  {
+    return !mNormalTimeouts.IsEmpty() ||
+           !mTrackingTimeouts.IsEmpty();
+  }
 
   nsresult SetTimeout(nsITimeoutHandler* aHandler,
                       int32_t interval, bool aIsInterval,
                       mozilla::dom::Timeout::Reason aReason,
                       int32_t* aReturn);
   void ClearTimeout(int32_t aTimerId,
                     mozilla::dom::Timeout::Reason aReason);
 
@@ -78,36 +84,43 @@ public:
   void Suspend();
   void Resume();
   void Freeze();
   void Thaw();
 
   // Initialize TimeoutManager before the first time it is accessed.
   static void Initialize();
 
-  // Run some code for each Timeout in our list.
+  // Exposed only for testing
+  bool IsTimeoutTracking(uint32_t aTimeoutId);
+
+  // Run some code for each Timeout in our list.  Note that this function
+  // doesn't guarantee that Timeouts are iterated in any particular order.
   template <class Callable>
-  void ForEachTimeout(Callable c)
+  void ForEachUnorderedTimeout(Callable c)
   {
-    mTimeouts.ForEach(c);
+    mNormalTimeouts.ForEach(c);
+    mTrackingTimeouts.ForEach(c);
   }
 
-  // Run some code for each Timeout in our list, but let the callback cancel
-  // the iteration by returning true.
+  // Run some code for each Timeout in our list, but let the callback cancel the
+  // iteration by returning true.  Note that this function doesn't guarantee
+  // that Timeouts are iterated in any particular order.
   template <class Callable>
-  void ForEachTimeoutAbortable(Callable c)
+  void ForEachUnorderedTimeoutAbortable(Callable c)
   {
-    mTimeouts.ForEachAbortable(c);
+    if (!mNormalTimeouts.ForEachAbortable(c)) {
+      mTrackingTimeouts.ForEachAbortable(c);
+    }
   }
 
 private:
   nsresult ResetTimersForThrottleReduction(int32_t aPreviousThrottleDelayMS);
 
 private:
-  typedef mozilla::LinkedList<mozilla::dom::Timeout> TimeoutList;
   struct Timeouts {
     Timeouts()
       : mTimeoutInsertionPoint(nullptr)
     {
     }
 
     // Insert aTimeout into the list, before all timeouts that would
     // fire after it, but no earlier than mTimeoutInsertionPoint, if any.
@@ -144,44 +157,55 @@ private:
     {
       for (Timeout* timeout = GetFirst();
            timeout;
            timeout = timeout->getNext()) {
         c(timeout);
       }
     }
 
+    // Returns true when a callback aborts iteration.
     template <class Callable>
-    void ForEachAbortable(Callable c)
+    bool ForEachAbortable(Callable c)
     {
       for (Timeout* timeout = GetFirst();
            timeout;
            timeout = timeout->getNext()) {
         if (c(timeout)) {
-          break;
+          return true;
         }
       }
+      return false;
     }
 
+    friend class OrderedTimeoutIterator;
+
   private:
+    typedef mozilla::LinkedList<mozilla::dom::Timeout> TimeoutList;
+
     // mTimeoutList is generally sorted by mWhen, unless mTimeoutInsertionPoint is
     // non-null.  In that case, the dummy timeout pointed to by
     // mTimeoutInsertionPoint may have a later mWhen than some of the timeouts
     // that come after it.
     TimeoutList               mTimeoutList;
     // If mTimeoutInsertionPoint is non-null, insertions should happen after it.
     // This is a dummy timeout at the moment; if that ever changes, the logic in
     // ResetTimersForThrottleReduction needs to change.
     mozilla::dom::Timeout*    mTimeoutInsertionPoint;
   };
 
+  friend class OrderedTimeoutIterator;
+
   // Each nsGlobalWindow object has a TimeoutManager member.  This reference
   // points to that holder object.
   nsGlobalWindow&             mWindow;
-  Timeouts                    mTimeouts;
+  // The list of timeouts coming from non-tracking scripts.
+  Timeouts                    mNormalTimeouts;
+  // The list of timeouts coming from scripts on the tracking protection list.
+  Timeouts                    mTrackingTimeouts;
   uint32_t                    mTimeoutIdCounter;
   uint32_t                    mTimeoutFiringDepth;
   mozilla::dom::Timeout*      mRunningTimeout;
 
    // The current idle request callback timeout handle
   uint32_t                    mIdleCallbackTimeoutCounter;
 
   int32_t                     mBackPressureDelayMS;

--- a/dom/base/nsDOMWindowUtils.cpp
+++ b/dom/base/nsDOMWindowUtils.cpp
@@ -106,16 +106,17 @@
 #include "nsNetUtil.h"
 #include "nsDocument.h"
 #include "HTMLImageElement.h"
 #include "mozilla/css/ImageLoader.h"
 #include "mozilla/layers/APZCTreeManager.h" // for layers::ZoomToRectBehavior
 #include "mozilla/dom/Promise.h"
 #include "mozilla/StyleSheetInlines.h"
 #include "mozilla/gfx/GPUProcessManager.h"
+#include "mozilla/dom/TimeoutManager.h"
 
 #ifdef XP_WIN
 #undef GetClassName
 #endif
 
 using namespace mozilla;
 using namespace mozilla::dom;
 using namespace mozilla::ipc;
@@ -4087,16 +4088,31 @@ nsDOMWindowUtils::GetGpuProcessPid(int32
     *aPid = pm->GPUProcessPid();
   } else {
     *aPid = -1;
   }
 
   return NS_OK;
 }
 
+NS_IMETHODIMP
+nsDOMWindowUtils::IsTimeoutTracking(uint32_t aTimeoutId, bool* aResult)
+{
+  NS_ENSURE_ARG_POINTER(aResult);
+  *aResult = false;
+
+  nsCOMPtr<nsPIDOMWindowOuter> window = do_QueryReferent(mWindow);
+  NS_ENSURE_STATE(window);
+  nsCOMPtr<nsPIDOMWindowInner> innerWindow = window->GetCurrentInnerWindow();
+  NS_ENSURE_STATE(innerWindow);
+
+  *aResult = innerWindow->TimeoutManager().IsTimeoutTracking(aTimeoutId);
+  return NS_OK;
+}
+
 NS_INTERFACE_MAP_BEGIN(nsTranslationNodeList)
   NS_INTERFACE_MAP_ENTRY(nsISupports)
   NS_INTERFACE_MAP_ENTRY(nsITranslationNodeList)
 NS_INTERFACE_MAP_END
 
 NS_IMPL_ADDREF(nsTranslationNodeList)
 NS_IMPL_RELEASE(nsTranslationNodeList)
 

--- a/dom/base/nsGlobalWindow.cpp
+++ b/dom/base/nsGlobalWindow.cpp
@@ -1931,17 +1931,17 @@ NS_IMPL_CYCLE_COLLECTION_TRAVERSE_BEGIN_
   NS_IMPL_CYCLE_COLLECTION_TRAVERSE(mSpeechSynthesis)
 #endif
 
   NS_IMPL_CYCLE_COLLECTION_TRAVERSE(mOuterWindow)
 
   NS_IMPL_CYCLE_COLLECTION_TRAVERSE(mListenerManager)
 
   if (tmp->mTimeoutManager) {
-    tmp->mTimeoutManager->ForEachTimeout([&cb](Timeout* timeout) {
+    tmp->mTimeoutManager->ForEachUnorderedTimeout([&cb](Timeout* timeout) {
       cb.NoteNativeChild(timeout, NS_CYCLE_COLLECTION_PARTICIPANT(Timeout));
     });
   }
 
   NS_IMPL_CYCLE_COLLECTION_TRAVERSE(mLocation)
   NS_IMPL_CYCLE_COLLECTION_TRAVERSE(mHistory)
   NS_IMPL_CYCLE_COLLECTION_TRAVERSE(mCustomElements)
 

--- a/dom/base/nsImageLoadingContent.cpp
+++ b/dom/base/nsImageLoadingContent.cpp
@@ -39,17 +39,16 @@
 #include "nsLayoutUtils.h"
 #include "nsIContentPolicy.h"
 #include "nsSVGEffects.h"
 
 #include "gfxPrefs.h"
 
 #include "mozAutoDocUpdate.h"
 #include "mozilla/AsyncEventDispatcher.h"
-#include "mozilla/CycleCollectedJSContext.h"
 #include "mozilla/EventStates.h"
 #include "mozilla/dom/Element.h"
 #include "mozilla/dom/ImageTracker.h"
 #include "mozilla/dom/ScriptSettings.h"
 #include "mozilla/Preferences.h"
 
 #ifdef LoadImage
 // Undefine LoadImage to prevent naming conflict with Windows.
@@ -144,19 +143,16 @@ nsImageLoadingContent::Notify(imgIReques
     // We should definitely have a request here
     MOZ_ASSERT(aRequest, "no request?");
 
     NS_PRECONDITION(aRequest == mCurrentRequest || aRequest == mPendingRequest,
                     "Unknown request");
   }
 
   {
-    MOZ_RELEASE_ASSERT(js::AllowGCBarriers(CycleCollectedJSContext::Get()->Context()),
-                       "ImageObservers can be implement in JS, so they should not be called during painting. See bug 1311841");
-
     nsAutoScriptBlocker scriptBlocker;
 
     for (ImageObserver* observer = &mObserverList, *next; observer;
          observer = next) {
       next = observer->mNext;
       if (observer->mObserver) {
         observer->mObserver->Notify(aRequest, aType, aData);
       }

--- a/dom/base/test/file_timer_flood.html
+++ b/dom/base/test/file_timer_flood.html
@@ -1,19 +1,29 @@
 <!DOCTYPE HTML>
 <html>
 <body>
 <script>
 let count = 0;
-function cb() {
+let last_timer_set = 0;
+let last_timer_observed = 0;
+function cb(timer_observed) {
+  if (timer_observed > last_timer_observed) {
+    // In order to make the test more efficient, we don't use the SimpleTest
+    // ok() function to avoid generating one test assertion per one of these
+    // checks.  We only send a message to the parent which fails the test if
+    // we detect out of order firing of timeouts.
+    window.parent.postMessage('OUT_OF_ORDER', '*');
+  }
+  last_timer_observed = timer_observed;
   count += 1;
   // Notify our parent that we are ready once the timer flood has
   // warmed up.
   if (count === 10000) {
     window.parent.postMessage('STARTED', '*');
   }
-  setTimeout(cb, 0);
-  setTimeout(cb, 0);
+  last_timer_set = setTimeout(cb.bind(last_timer_set), 0);
+  last_timer_set = setTimeout(cb.bind(last_timer_set), 0);
 }
 addEventListener('load', cb);
 </script>
 </body>
 </html>

--- a/dom/base/test/test_timer_flood.html
+++ b/dom/base/test/test_timer_flood.html
@@ -8,32 +8,43 @@
 </head>
 <body>
 <p id="display"></p>
 <div id="content" style="display: none">
 </div>
 <pre id="test">
 <script type="application/javascript">
 SimpleTest.waitForExplicitFinish();
+// This test takes a long time to run and it times out on Android debug as a result.
+SimpleTest.requestLongerTimeout(5);
 
 function onLoad() {
   return new Promise(resolve => {
     addEventListener('load', resolve, { once: true });
   });
 }
 
+function setPrefs() {
+  // Put timeouts randomly in the tracking or normal buffer.  We do this in order to
+  // test to ensure that by default, this will not change the scheduling of timeouts.
+  return SpecialPowers.pushPrefEnv({"set": [["dom.timeout_bucketing_strategy", 3]]});
+}
+
 // Create a frame that executes a timer flood.  The frame signals
 // that is ready once the flood has had a chance to warm up.
 function withFloodFrame() {
-  return new Promise(resolve => {
+  return new Promise((resolve, reject) => {
     let frame = document.createElement('iframe');
     addEventListener('message', function onMsg(evt) {
       if (evt.data === 'STARTED') {
         removeEventListener('message', onMsg);
         resolve(frame);
+      } else if (evt.data == 'OUT_OF_ORDER') {
+        ok(false, "Out of order timeout observed");
+        reject();
       }
     });
     frame.src = 'file_timer_flood.html';
     document.body.appendChild(frame);
   });
 }
 
 // Test that we can load documents during a timer flood.
@@ -70,17 +81,19 @@ function testRequestAnimationFrame() {
       }
     };
     requestAnimationFrame(nextFrame);
   });
 }
 
 let floodFrame;
 
-onLoad().then(_ => {
+onLoad()
+.then(setPrefs)
+.then(_ => {
   // Start a timer flood in a frame.
   return withFloodFrame();
 }).then(frame => {
   floodFrame = frame;
 
   // Next we are going to start a bunch of asynchronous work that we
   // expect to complete in spite of the timer flood.  The type of work
   // is a bit arbitrary, but is chosen to reflect the kinds of things
@@ -104,13 +117,15 @@ onLoad().then(_ => {
 
   // Wait for all tests to finish.  If we do not handle the timer flood
   // well then this will likely time out.
   return Promise.all(tests);
 }).then(_ => {
   ok(true, 'completed tests without timing out');
   floodFrame.remove();
   SimpleTest.finish();
+}).catch(_ => {
+  SimpleTest.finish();
 });
 </script>
 </pre>
 </body>
 </html>

--- a/dom/canvas/WebGL2Context.h
+++ b/dom/canvas/WebGL2Context.h
@@ -265,18 +265,22 @@ public:
     // -------------------------------------------------------------------------
     // Writing to the drawing buffer
 
     /* Implemented in WebGLContext
     void VertexAttribDivisor(GLuint index, GLuint divisor);
     void DrawArraysInstanced(GLenum mode, GLint first, GLsizei count, GLsizei instanceCount);
     void DrawElementsInstanced(GLenum mode, GLsizei count, GLenum type, GLintptr offset, GLsizei instanceCount);
     */
-    void DrawRangeElements(GLenum mode, GLuint start, GLuint end, GLsizei count, GLenum type, GLintptr offset);
 
+    void DrawRangeElements(GLenum mode, GLuint start, GLuint end, GLsizei count,
+                           GLenum type, WebGLintptr byteOffset)
+    {
+        DrawElements(mode, count, type, byteOffset);
+    }
 
     // ------------------------------------------------------------------------
     // Multiple Render Targets - WebGL2ContextMRTs.cpp
     /* Implemented in WebGLContext
     void DrawBuffers(const dom::Sequence<GLenum>& buffers);
     */
 
 private:
@@ -320,16 +324,18 @@ public:
     void SamplerParameterf(WebGLSampler& sampler, GLenum pname, GLfloat param);
     void GetSamplerParameter(JSContext*, const WebGLSampler& sampler, GLenum pname,
                              JS::MutableHandleValue retval);
 
 
     // -------------------------------------------------------------------------
     // Sync objects - WebGL2ContextSync.cpp
 
+    const GLuint64 kMaxClientWaitSyncTimeoutNS = 1000 * 1000 * 1000; // 1000ms in ns.
+
     already_AddRefed<WebGLSync> FenceSync(GLenum condition, GLbitfield flags);
     bool IsSync(const WebGLSync* sync);
     void DeleteSync(WebGLSync* sync);
     GLenum ClientWaitSync(const WebGLSync& sync, GLbitfield flags, GLuint64 timeout);
     void WaitSync(const WebGLSync& sync, GLbitfield flags, GLint64 timeout);
     void GetSyncParameter(JSContext*, const WebGLSync& sync, GLenum pname,
                           JS::MutableHandleValue retval);
 

--- a/dom/canvas/WebGL2ContextBuffers.cpp
+++ b/dom/canvas/WebGL2ContextBuffers.cpp
@@ -126,18 +126,18 @@ WebGL2Context::GetBufferSubData(GLenum t
     }
     const GLsizeiptr glByteLen(byteLen);
 
     ////
 
     gl->MakeCurrent();
     const ScopedLazyBind readBind(gl, target, buffer);
 
-    const auto mappedBytes = gl->fMapBufferRange(target, srcByteOffset, glByteLen,
-                                                 LOCAL_GL_MAP_READ_BIT);
-    // Warning: Possibly shared memory.  See bug 1225033.
     if (byteLen) {
+        const auto mappedBytes = gl->fMapBufferRange(target, srcByteOffset, glByteLen,
+                                                     LOCAL_GL_MAP_READ_BIT);
+        // Warning: Possibly shared memory.  See bug 1225033.
         memcpy(bytes, mappedBytes, byteLen);
+        gl->fUnmapBuffer(target);
     }
-    gl->fUnmapBuffer(target);
 }
 
 } // namespace mozilla

deleted file mode 100644
--- a/dom/canvas/WebGL2ContextDraw.cpp
+++ /dev/null
@@ -1,19 +0,0 @@
-/* -*- Mode: C++; tab-width: 4; indent-tabs-mode: nil; c-basic-offset: 4 -*- */
-/* This Source Code Form is subject to the terms of the Mozilla Public
- * License, v. 2.0. If a copy of the MPL was not distributed with this
- * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
-
-#include "WebGL2Context.h"
-
-namespace mozilla {
-
-// -------------------------------------------------------------------------
-// Writing to the drawing buffer
-
-void
-WebGL2Context::DrawRangeElements(GLenum mode, GLuint start, GLuint end, GLsizei count, GLenum type, GLintptr offset)
-{
-    GenerateWarning("drawRangeElements: Not Implemented.");
-}
-
-} // namespace mozilla

--- a/dom/canvas/WebGL2ContextState.cpp
+++ b/dom/canvas/WebGL2ContextState.cpp
@@ -107,23 +107,23 @@ WebGL2Context::GetParameter(JSContext* c
       // value is 4 * GL_MAX_VARYING_VECTORS
       GLint val;
       gl->fGetIntegerv(LOCAL_GL_MAX_VARYING_VECTORS, &val);
       return JS::Int32Value(4*val);
     }
 
     /* GLint64 */
     case LOCAL_GL_MAX_CLIENT_WAIT_TIMEOUT_WEBGL:
-      return JS::NumberValue(0); // TODO
+      return JS::NumberValue(kMaxClientWaitSyncTimeoutNS);
 
     case LOCAL_GL_MAX_ELEMENT_INDEX:
       // GL_MAX_ELEMENT_INDEX becomes available in GL 4.3 or via ES3
       // compatibility
       if (!gl->IsSupported(gl::GLFeature::ES3_compatibility))
-        return JS::NumberValue(0);
+        return JS::NumberValue(UINT32_MAX);
 
       /*** fall through to fGetInteger64v ***/
       MOZ_FALLTHROUGH;
 
     case LOCAL_GL_MAX_COMBINED_FRAGMENT_UNIFORM_COMPONENTS:
     case LOCAL_GL_MAX_COMBINED_VERTEX_UNIFORM_COMPONENTS:
     case LOCAL_GL_MAX_UNIFORM_BLOCK_SIZE: {
       GLint64 val;

--- a/dom/canvas/WebGL2ContextSync.cpp
+++ b/dom/canvas/WebGL2ContextSync.cpp
@@ -62,16 +62,22 @@ WebGL2Context::ClientWaitSync(const WebG
     if (!ValidateObject(funcName, sync))
         return LOCAL_GL_WAIT_FAILED;
 
     if (flags != 0 && flags != LOCAL_GL_SYNC_FLUSH_COMMANDS_BIT) {
         ErrorInvalidValue("%s: `flags` must be SYNC_FLUSH_COMMANDS_BIT or 0.", funcName);
         return LOCAL_GL_WAIT_FAILED;
     }
 
+    if (timeout > kMaxClientWaitSyncTimeoutNS) {
+        ErrorInvalidOperation("%s: `timeout` must not exceed %s nanoseconds.", funcName,
+                              "MAX_CLIENT_WAIT_TIMEOUT_WEBGL");
+        return LOCAL_GL_WAIT_FAILED;
+    }
+
     MakeContextCurrent();
     return gl->fClientWaitSync(sync.mGLName, flags, timeout);
 }
 
 void
 WebGL2Context::WaitSync(const WebGLSync& sync, GLbitfield flags, GLint64 timeout)
 {
     const char funcName[] = "waitSync";

--- a/dom/canvas/WebGL2ContextUniforms.cpp
+++ b/dom/canvas/WebGL2ContextUniforms.cpp
@@ -177,16 +177,24 @@ WebGL2Context::GetActiveUniforms(JSConte
         return;
 
     if (!ValidateUniformEnum(this, pname, funcName))
         return;
 
     if (!ValidateObject("getActiveUniforms: program", program))
         return;
 
+    const auto& numActiveUniforms = program.LinkInfo()->uniforms.size();
+    for (const auto& curIndex : uniformIndices) {
+        if (curIndex >= numActiveUniforms) {
+            ErrorInvalidValue("%s: Too-large active uniform index queried.", funcName);
+            return;
+        }
+    }
+
     const auto& count = uniformIndices.Length();
 
     JS::Rooted<JSObject*> array(cx, JS_NewArrayObject(cx, count));
     UniquePtr<GLint[]> samples(new GLint[count]);
     if (!array || !samples) {
         ErrorOutOfMemory("%s: Failed to allocate buffers.", funcName);
         return;
     }

--- a/dom/canvas/WebGLContext.h
+++ b/dom/canvas/WebGLContext.h
@@ -969,16 +969,17 @@ public:
 
 private:
     // State tracking slots
     realGLboolean mDitherEnabled;
     realGLboolean mRasterizerDiscardEnabled;
     realGLboolean mScissorTestEnabled;
     realGLboolean mDepthTestEnabled;
     realGLboolean mStencilTestEnabled;
+    GLenum mGenerateMipmapHint;
 
     bool ValidateCapabilityEnum(GLenum cap, const char* info);
     realGLboolean* GetStateTrackingSlot(GLenum cap);
 
 // -----------------------------------------------------------------------------
 // Texture funcions (WebGLContextTextures.cpp)
 public:
     void ActiveTexture(GLenum texUnit);

--- a/dom/canvas/WebGLContextGL.cpp
+++ b/dom/canvas/WebGLContextGL.cpp
@@ -687,19 +687,16 @@ WebGLContext::GetFramebufferAttachmentPa
         default:
             ErrorInvalidEnum("%s: With the default framebuffer, can only query COLOR, DEPTH,"
                              " or STENCIL for GL_FRAMEBUFFER_ATTACHMENT_OBJECT_TYPE",
                              funcName);
             return JS::NullValue();
         }
         return JS::Int32Value(LOCAL_GL_FRAMEBUFFER_DEFAULT);
 
-    case LOCAL_GL_FRAMEBUFFER_ATTACHMENT_OBJECT_NAME:
-        return JS::NullValue();
-
     ////////////////
 
     case LOCAL_GL_FRAMEBUFFER_ATTACHMENT_RED_SIZE:
     case LOCAL_GL_FRAMEBUFFER_ATTACHMENT_GREEN_SIZE:
     case LOCAL_GL_FRAMEBUFFER_ATTACHMENT_BLUE_SIZE:
         if (attachment == LOCAL_GL_BACK)
             return JS::NumberValue(8);
         return JS::NumberValue(0);
@@ -942,16 +939,18 @@ WebGLContext::Hint(GLenum target, GLenum
 {
     if (IsContextLost())
         return;
 
     bool isValid = false;
 
     switch (target) {
     case LOCAL_GL_GENERATE_MIPMAP_HINT:
+        mGenerateMipmapHint = mode;
+
         // Deprecated and removed in desktop GL Core profiles.
         if (gl->IsCoreProfile())
             return;
 
         isValid = true;
         break;
 
     case LOCAL_GL_FRAGMENT_SHADER_DERIVATIVE_HINT:
@@ -1460,17 +1459,17 @@ ValidateReadPixelsFormatAndType(const we
     }
 
     if (pi.type == LOCAL_GL_UNSIGNED_INT_24_8) {
         webgl->ErrorInvalidEnum("readPixels: Invalid type: 0x%04x", pi.type);
         return false;
     }
 
     MOZ_ASSERT(gl->IsCurrent());
-    if (gl->IsSupported(gl::GLFeature::ES2_compatibility)) {
+    if (gl->IsGLES()) {
         const auto auxFormat = gl->GetIntAs<GLenum>(LOCAL_GL_IMPLEMENTATION_COLOR_READ_FORMAT);
         const auto auxType = gl->GetIntAs<GLenum>(LOCAL_GL_IMPLEMENTATION_COLOR_READ_TYPE);
 
         if (auxFormat && auxType &&
             pi.format == auxFormat && pi.type == auxType)
         {
             return true;
         }

--- a/dom/canvas/WebGLContextState.cpp
+++ b/dom/canvas/WebGLContextState.cpp
@@ -381,45 +381,53 @@ WebGLContext::GetParameter(JSContext* cx
         case LOCAL_GL_STENCIL_BACK_PASS_DEPTH_FAIL:
         case LOCAL_GL_STENCIL_BACK_PASS_DEPTH_PASS:
         case LOCAL_GL_DEPTH_FUNC:
         case LOCAL_GL_BLEND_SRC_RGB:
         case LOCAL_GL_BLEND_SRC_ALPHA:
         case LOCAL_GL_BLEND_DST_RGB:
         case LOCAL_GL_BLEND_DST_ALPHA:
         case LOCAL_GL_BLEND_EQUATION_RGB:
-        case LOCAL_GL_BLEND_EQUATION_ALPHA:
-        case LOCAL_GL_GENERATE_MIPMAP_HINT: {
+        case LOCAL_GL_BLEND_EQUATION_ALPHA: {
             GLint i = 0;
             gl->fGetIntegerv(pname, &i);
             return JS::NumberValue(uint32_t(i));
         }
+
+        case LOCAL_GL_GENERATE_MIPMAP_HINT:
+            return JS::NumberValue(mGenerateMipmapHint);
+
         case LOCAL_GL_IMPLEMENTATION_COLOR_READ_TYPE: {
             const webgl::FormatUsageInfo* usage;
             uint32_t width, height;
             if (!ValidateCurFBForRead(funcName, &usage, &width, &height))
                 return JS::NullValue();
 
             GLint i = 0;
-            if (gl->IsSupported(gl::GLFeature::ES2_compatibility)) {
+            if (gl->IsGLES()) {
+                // ES2_compatibility always returns UNSIGNED_BYTE here, so
+                // branch on actual IsGLES().
+                // Also OSX+NV generates an error here.
                 gl->fGetIntegerv(pname, &i);
             } else {
                 i = LOCAL_GL_UNSIGNED_BYTE;
             }
-
             return JS::NumberValue(uint32_t(i));
         }
         case LOCAL_GL_IMPLEMENTATION_COLOR_READ_FORMAT: {
             const webgl::FormatUsageInfo* usage;
             uint32_t width, height;
             if (!ValidateCurFBForRead(funcName, &usage, &width, &height))
                 return JS::NullValue();
 
             GLint i = 0;
-            if (gl->IsSupported(gl::GLFeature::ES2_compatibility)) {
+            if (gl->IsGLES()) {
+                // ES2_compatibility always returns UNSIGNED_BYTE here, so
+                // branch on actual IsGLES().
+                // Also OSX+NV generates an error here.
                 gl->fGetIntegerv(pname, &i);
             } else {
                 i = LOCAL_GL_RGBA;
             }
 
             // OpenGL ES 3.0.4 p112 Table 3.2 shows that read format SRGB_ALPHA is
             // not supported. And if internal format of fbo is SRGB8_ALPHA8, then
             // IMPLEMENTATION_COLOR_READ_FORMAT is SRGB_ALPHA which is not supported

--- a/dom/canvas/WebGLContextValidate.cpp
+++ b/dom/canvas/WebGLContextValidate.cpp
@@ -496,16 +496,17 @@ WebGLContext::InitAndValidateGL(FailureR
     AssertUintParamCorrect(gl, LOCAL_GL_STENCIL_VALUE_MASK,      mStencilValueMaskFront);
     AssertUintParamCorrect(gl, LOCAL_GL_STENCIL_BACK_VALUE_MASK, mStencilValueMaskBack);
     AssertUintParamCorrect(gl, LOCAL_GL_STENCIL_WRITEMASK,       mStencilWriteMaskFront);
     AssertUintParamCorrect(gl, LOCAL_GL_STENCIL_BACK_WRITEMASK,  mStencilWriteMaskBack);
 
     mDitherEnabled = true;
     mRasterizerDiscardEnabled = false;
     mScissorTestEnabled = false;
+    mGenerateMipmapHint = LOCAL_GL_DONT_CARE;
 
     // Bindings, etc.
     mActiveTexture = 0;
     mDefaultFB_DrawBuffer0 = LOCAL_GL_BACK;
 
     mEmitContextLostErrorOnce = true;
     mWebGLError = LOCAL_GL_NO_ERROR;
     mUnderlyingGLError = LOCAL_GL_NO_ERROR;

--- a/dom/canvas/WebGLTexture.cpp
+++ b/dom/canvas/WebGLTexture.cpp
@@ -233,26 +233,36 @@ WebGLTexture::IsMipmapComplete(uint32_t 
                 cur.mFormat != baseImageInfo.mFormat)
             {
                 return false;
             }
         }
 
         // GLES 3.0.4, p158:
         // "[...] until the last array is reached with dimension 1 x 1 x 1."
-        if (refWidth == 1 &&
-            refHeight == 1 &&
-            refDepth == 1)
-        {
-            break;
+        if (mTarget == LOCAL_GL_TEXTURE_3D) {
+            if (refWidth == 1 &&
+                refHeight == 1 &&
+                refDepth == 1)
+            {
+                break;
+            }
+
+            refDepth = std::max(uint32_t(1), refDepth / 2);
+        } else {
+            // TEXTURE_2D_ARRAY may have depth != 1, but that's normal.
+            if (refWidth == 1 &&
+                refHeight == 1)
+            {
+                break;
+            }
         }
 
         refWidth  = std::max(uint32_t(1), refWidth  / 2);
         refHeight = std::max(uint32_t(1), refHeight / 2);
-        refDepth  = std::max(uint32_t(1), refDepth  / 2);
     }
 
     return true;
 }
 
 bool
 WebGLTexture::IsCubeComplete() const
 {
@@ -1009,17 +1019,17 @@ WebGLTexture::IsTexture() const
 // See this discussion:
 //   https://www.khronos.org/webgl/public-mailing-list/archives/1008/msg00014.html
 void
 WebGLTexture::TexParameter(TexTarget texTarget, GLenum pname, GLint* maybeIntParam,
                            GLfloat* maybeFloatParam)
 {
     MOZ_ASSERT(maybeIntParam || maybeFloatParam);
 
-    GLint   intParam   = maybeIntParam   ? *maybeIntParam   : GLint(*maybeFloatParam);
+    GLint   intParam   = maybeIntParam   ? *maybeIntParam   : GLint(roundf(*maybeFloatParam));
     GLfloat floatParam = maybeFloatParam ? *maybeFloatParam : GLfloat(*maybeIntParam);
 
     bool isPNameValid = false;
     switch (pname) {
     // GLES 2.0.25 p76:
     case LOCAL_GL_TEXTURE_WRAP_S:
     case LOCAL_GL_TEXTURE_WRAP_T:
     case LOCAL_GL_TEXTURE_MIN_FILTER:

--- a/dom/canvas/WebGLTextureUpload.cpp
+++ b/dom/canvas/WebGLTextureUpload.cpp
@@ -439,16 +439,21 @@ WebGLTexture::TexSubImage(const char* fu
 {
     const GLint border = 0;
     dom::RootedTypedArray<dom::Uint8ClampedArray> scopedArr(dom::RootingCx());
     const auto blob = ValidateTexOrSubImage(mContext, funcName, target, width, height,
                                             depth, border, pi, src, &scopedArr);
     if (!blob)
         return;
 
+    if (!blob->HasData()) {
+        mContext->ErrorInvalidValue("%s: Source must not be null.", funcName);
+        return;
+    }
+
     TexSubImage(funcName, target, level, xOffset, yOffset, zOffset, pi, blob.get());
 }
 
 //////////////////////////////////////////////////////////////////////////////////////////
 //////////////////////////////////////////////////////////////////////////////////////////
 
 static bool
 ValidateTexImage(WebGLContext* webgl, WebGLTexture* texture, const char* funcName,
@@ -2014,16 +2019,23 @@ DoCopyTexOrSubImage(WebGLContext* webgl,
         return true;
     } while (false);
 
     if (error == LOCAL_GL_OUT_OF_MEMORY) {
         webgl->ErrorOutOfMemory("%s: Ran out of memory during texture copy.", funcName);
         return false;
     }
 
+    if (gl->IsANGLE() && error == LOCAL_GL_INVALID_OPERATION) {
+        webgl->ErrorImplementationBug("%s: ANGLE is particular about CopyTexSubImage"
+                                      " formats matching exactly.",
+                                      funcName);
+        return false;
+    }
+
     MOZ_RELEASE_ASSERT(false, "GFX: We should have caught all other errors.");
     webgl->GenerateWarning("%s: Unexpected error during texture copy. Context lost.",
                            funcName);
     webgl->ForceLoseContext();
     return false;
 }
 
 // There is no CopyTexImage3D.

--- a/dom/canvas/moz.build
+++ b/dom/canvas/moz.build
@@ -68,17 +68,16 @@ SOURCES += [
 
 # WebGL Sources
 UNIFIED_SOURCES += [
     'TexUnpackBlob.cpp',
     'WebGL1Context.cpp',
     'WebGL1ContextUniforms.cpp',
     'WebGL2Context.cpp',
     'WebGL2ContextBuffers.cpp',
-    'WebGL2ContextDraw.cpp',
     'WebGL2ContextFramebuffers.cpp',
     'WebGL2ContextMRTs.cpp',
     'WebGL2ContextPrograms.cpp',
     'WebGL2ContextQueries.cpp',
     'WebGL2ContextRenderbuffers.cpp',
     'WebGL2ContextSamplers.cpp',
     'WebGL2ContextState.cpp',
     'WebGL2ContextSync.cpp',

--- a/dom/canvas/test/webgl-conf/generated-mochitest.ini
+++ b/dom/canvas/test/webgl-conf/generated-mochitest.ini
@@ -4581,17 +4581,16 @@ skip-if = (os == 'android' || os == 'lin
 skip-if = (os == 'android' || os == 'linux' || (os == 'win' && os_version == '5.1'))
 [generated/test_2_conformance2__samplers__sampler-drawing-test.html]
 skip-if = (os == 'android' || os == 'linux' || (os == 'win' && os_version == '5.1'))
 [generated/test_2_conformance2__samplers__samplers.html]
 skip-if = (os == 'android' || os == 'linux' || (os == 'win' && os_version == '5.1'))
 [generated/test_2_conformance2__state__gl-enum-tests.html]
 skip-if = (os == 'android' || os == 'linux' || (os == 'win' && os_version == '5.1'))
 [generated/test_2_conformance2__state__gl-get-calls.html]
-fail-if = (os == 'mac')
 skip-if = (os == 'android' || os == 'linux' || (os == 'win' && os_version == '5.1'))
 [generated/test_2_conformance2__state__gl-getstring.html]
 skip-if = (os == 'android' || os == 'linux' || (os == 'win' && os_version == '5.1'))
 [generated/test_2_conformance2__state__gl-object-get-calls.html]
 skip-if = (os == 'android' || os == 'linux' || (os == 'win' && os_version == '5.1'))
 [generated/test_2_conformance2__transform_feedback__transform_feedback.html]
 skip-if = (os == 'android' || os == 'linux' || (os == 'win' && os_version == '5.1'))
 [generated/test_2_conformance2__vertex_arrays__vertex-array-object.html]
@@ -5764,17 +5763,17 @@ skip-if = (os == 'android' || os == 'lin
 skip-if = (os == 'android' || os == 'linux' || (os == 'win' && os_version == '5.1'))
 [generated/test_2_conformance__rendering__gl-scissor-test.html]
 skip-if = (os == 'android' || os == 'linux' || (os == 'win' && os_version == '5.1'))
 [generated/test_2_conformance__rendering__gl-viewport-test.html]
 skip-if = (os == 'android' || os == 'linux' || (os == 'win' && os_version == '5.1'))
 [generated/test_2_conformance__rendering__line-loop-tri-fan.html]
 skip-if = (os == 'android' || os == 'linux' || (os == 'win' && os_version == '5.1'))
 [generated/test_2_conformance__rendering__many-draw-calls.html]
-skip-if = (os == 'android' || os == 'linux' || (os == 'win' && os_version == '5.1'))
+skip-if = debug || (os == 'android' || os == 'linux' || (os == 'win' && os_version == '5.1'))
 [generated/test_2_conformance__rendering__more-than-65536-indices.html]
 skip-if = (os == 'android' || os == 'linux' || (os == 'win' && os_version == '5.1'))
 [generated/test_2_conformance__rendering__multisample-corruption.html]
 skip-if = (os == 'android' || os == 'linux' || (os == 'win' && os_version == '5.1'))
 [generated/test_2_conformance__rendering__negative-one-index.html]
 fail-if = (os == 'mac')
 skip-if = (os == 'android' || os == 'linux' || (os == 'win' && os_version == '5.1'))
 [generated/test_2_conformance__rendering__point-no-attributes.html]
@@ -5789,18 +5788,17 @@ skip-if = (os == 'android' || os == 'lin
 skip-if = (os == 'android' || os == 'linux' || (os == 'win' && os_version == '5.1'))
 [generated/test_2_conformance__rendering__simple.html]
 skip-if = (os == 'android' || os == 'linux' || (os == 'win' && os_version == '5.1'))
 [generated/test_2_conformance__rendering__triangle.html]
 skip-if = (os == 'android' || os == 'linux' || (os == 'win' && os_version == '5.1'))
 [generated/test_2_conformance__state__gl-enable-enum-test.html]
 skip-if = (os == 'android' || os == 'linux' || (os == 'win' && os_version == '5.1'))
 [generated/test_2_conformance__state__gl-get-calls.html]
-skip-if = (os == 'mac' && debug) || (os == 'android' || os == 'linux' || (os == 'win' && os_version == '5.1'))
-fail-if = (os == 'mac')
+skip-if = (os == 'android' || os == 'linux' || (os == 'win' && os_version == '5.1'))
 [generated/test_2_conformance__state__gl-geterror.html]
 skip-if = (os == 'android' || os == 'linux' || (os == 'win' && os_version == '5.1'))
 [generated/test_2_conformance__state__gl-initial-state.html]
 skip-if = (os == 'android' || os == 'linux' || (os == 'win' && os_version == '5.1'))
 [generated/test_2_conformance__state__state-uneffected-after-compositing.html]
 skip-if = (os == 'android' || os == 'linux' || (os == 'win' && os_version == '5.1'))
 [generated/test_2_conformance__textures__misc__copy-tex-image-2d-formats.html]
 skip-if = (os == 'mac') || (os == 'android' || os == 'linux' || (os == 'win' && os_version == '5.1'))

--- a/dom/canvas/test/webgl-conf/mochitest-errata.ini
+++ b/dom/canvas/test/webgl-conf/mochitest-errata.ini
@@ -296,16 +296,19 @@ skip-if = (os == 'android')
 skip-if = (os == 'android')
 [generated/test_conformance__glsl__variables__gl-fragcoord.html]
 # Crashes
 skip-if = (os == 'android')
 [generated/test_conformance__rendering__many-draw-calls.html]
 # Crashes on Android
 # Times-out on DEBUG builds
 skip-if = (os == 'android') || debug
+[generated/test_2_conformance__rendering__many-draw-calls.html]
+# Appears to just take too long on debug, most of the time.
+skip-if = debug
 [generated/test_conformance__uniforms__out-of-bounds-uniform-array-access.html]
 # Crashes
 skip-if = (os == 'android') || (os == 'mac' && os_version == '10.6')
 [generated/test_conformance__glsl__samplers__glsl-function-texture2dproj.html]
 # Crashes
 skip-if = (os == 'android')
 [generated/test_conformance__rendering__framebuffer-switch.html]
 # Crashes
@@ -528,35 +531,27 @@ fail-if = (os == 'mac' && os_version == 
 fail-if = (os == 'mac' && os_version == '10.8')
 [generated/test_conformance__glsl__variables__gl-pointcoord.html]
 fail-if = (os == 'mac' && os_version == '10.8')
 [generated/test_conformance__limits__gl-max-texture-dimensions.html]
 fail-if = (os == 'mac' && os_version == '10.8')
 
 ####################
 # failure on OSX
-[generated/test_2_conformance2__state__gl-get-calls.html]
-fail-if = (os == 'mac')
 [generated/test_conformance__extensions__angle-instanced-arrays.html]
 fail-if = (os == 'mac')
 [generated/test_conformance__glsl__misc__shaders-with-invariance.html]
 fail-if = (os == 'mac')
 
 [generated/test_2_conformance2__extensions__ext-color-buffer-float.html]
 skip-if = (os == 'mac' && debug)
 [generated/test_2_conformance__limits__gl-line-width.html]
 skip-if = (os == 'mac')
 [generated/test_2_conformance__misc__type-conversion-test.html]
 skip-if = (os == 'mac' && debug)
-[generated/test_2_conformance__state__gl-get-calls.html]
-# Hit MOZ_GL_DEBUG_ABORT_ON_ERROR on debug build
-fail-if = (os == 'mac')
-skip-if = (os == 'mac' && debug)
-
-
 
 ########################################################################
 ########################################################################
 # Win
 [generated/test_2_conformance__ogles__GL__built_in_varying_array_out_of_bounds__built_in_varying_array_out_of_bounds_001_to_001.html]
 # time out crash
 skip-if = (os == 'win')
 [generated/test_conformance__ogles__GL__built_in_varying_array_out_of_bounds__built_in_varying_array_out_of_bounds_001_to_001.html]

--- a/dom/events/EventStateManager.cpp
+++ b/dom/events/EventStateManager.cpp
@@ -3322,16 +3322,30 @@ EventStateManager::PostHandleEvent(nsPre
     }
     break;
 
   case eDragEnter:
   case eDragOver:
     {
       NS_ASSERTION(aEvent->mClass == eDragEventClass, "Expected a drag event");
 
+      // Check if the drag is occurring inside a scrollable area. If so, scroll
+      // the area when the mouse is near the edges.
+      if (mCurrentTarget && aEvent->mMessage == eDragOver) {
+        nsIFrame* checkFrame = mCurrentTarget;
+        while (checkFrame) {
+          nsIScrollableFrame* scrollFrame = do_QueryFrame(checkFrame);
+          // Break out so only the innermost scrollframe is scrolled.
+          if (scrollFrame && scrollFrame->DragScroll(aEvent)) {
+            break;
+          }
+          checkFrame = checkFrame->GetParent();
+        }
+      }
+
       nsCOMPtr<nsIDragSession> dragSession = nsContentUtils::GetDragSession();
       if (!dragSession)
         break;
 
       // Reset the flag.
       dragSession->SetOnlyChromeDrop(false);
       if (mPresContext) {
         EnsureDocument(mPresContext);

--- a/dom/interfaces/base/nsIDOMWindowUtils.idl
+++ b/dom/interfaces/base/nsIDOMWindowUtils.idl
@@ -1968,16 +1968,22 @@ interface nsIDOMWindowUtils : nsISupport
    */
   void terminateGPUProcess();
 
   /**
     * Returns the GPU process pid, or -1 if there is no GPU process.
     */
   readonly attribute int32_t gpuProcessPid;
 
+  /**
+   * Returns true if the given timeout ID is in the list of tracking
+   * timeouts.
+   */
+  boolean isTimeoutTracking(in unsigned long timeoutId);
+
   // Match WidgetMouseEventBase::buttonType.
   const long MOUSE_BUTTON_LEFT_BUTTON   = 0;
   const long MOUSE_BUTTON_MIDDLE_BUTTON = 1;
   const long MOUSE_BUTTON_RIGHT_BUTTON  = 2;
 
   // Match WidgetMouseEventBase::buttonsFlag.
   const long MOUSE_BUTTONS_NO_BUTTON = 0x00;
   const long MOUSE_BUTTONS_LEFT_BUTTON = 0x01;

--- a/dom/media/gtest/TestMP3Demuxer.cpp
+++ b/dom/media/gtest/TestMP3Demuxer.cpp
@@ -265,16 +265,54 @@ protected:
       res.mDemuxer = new MP3TrackDemuxer(res.mResource);
       mTargets.push_back(res);
 
       streamRes.mResource = new MockMP3StreamMediaResource(streamRes.mFilePath);
       streamRes.mDemuxer = new MP3TrackDemuxer(streamRes.mResource);
       mTargets.push_back(streamRes);
     }
 
+    {
+      MP3Resource res;
+      res.mFilePath = "small-shot-partial-xing.mp3";
+      res.mIsVBR = true;
+      res.mFileSize = 6825;
+      res.mMPEGLayer = 3;
+      res.mMPEGVersion = 1;
+      res.mID3MajorVersion = 4;
+      res.mID3MinorVersion = 0;
+      res.mID3Flags = 0;
+      res.mID3Size = 24;
+      res.mDuration = 336686;
+      res.mDurationError = 0.01f;
+      res.mSeekError = 0.2f;
+      res.mSampleRate = 44100;
+      res.mSamplesPerFrame = 1152;
+      res.mNumSamples = 12;
+      res.mNumTrailingFrames = 0;
+      res.mBitrate = 256000;
+      res.mSlotSize = 1;
+      res.mPrivate = 0;
+      const int syncs[] = { 34, 556, 1078, 1601, 2123, 2646, 3168, 3691, 4213,
+                            4736, 5258, 5781, 6303 };
+      res.mSyncOffsets.insert(res.mSyncOffsets.begin(), syncs, syncs + 13);
+
+      // No content length can be estimated for CBR stream resources.
+      MP3Resource streamRes = res;
+      streamRes.mFileSize = -1;
+
+      res.mResource = new MockMP3MediaResource(res.mFilePath);
+      res.mDemuxer = new MP3TrackDemuxer(res.mResource);
+      mTargets.push_back(res);
+
+      streamRes.mResource = new MockMP3StreamMediaResource(streamRes.mFilePath);
+      streamRes.mDemuxer = new MP3TrackDemuxer(streamRes.mResource);
+      mTargets.push_back(streamRes);
+    }
+
     for (auto& target: mTargets) {
       ASSERT_EQ(NS_OK, target.mResource->Open(nullptr));
       ASSERT_TRUE(target.mDemuxer->Init());
     }
   }
 
   std::vector<MP3Resource> mTargets;
 };

--- a/dom/media/gtest/moz.build
+++ b/dom/media/gtest/moz.build
@@ -46,16 +46,17 @@ TEST_HARNESS_FILES.gtest += [
     'id3v2header.mp3',
     'mediasource_test.mp4',
     'negative_duration.mp4',
     'noise.mp3',
     'noise_vbr.mp3',
     'short-zero-in-moov.mp4',
     'short-zero-inband.mov',
     'small-shot-false-positive.mp3',
+    'small-shot-partial-xing.mp3',
     'small-shot.mp3',
     'test.webm',
     'test_case_1224361.vp8.ivf',
     'test_case_1224363.vp8.ivf',
     'test_case_1224369.vp8.ivf',
 ]
 
 include('/ipc/chromium/chromium-config.mozbuild')

new file mode 100644
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A9RL6T

--- a/gfx/2d/SourceSurfaceSkia.cpp
+++ b/gfx/2d/SourceSurfaceSkia.cpp
@@ -111,20 +111,21 @@ SourceSurfaceSkia::GetData()
     if (size.isValid()) {
       if (sk_sp<SkData> data = SkData::MakeUninitialized(size.value())) {
         SkImageInfo info = MakeSkiaImageInfo(mSize, mFormat);
         if (mImage->readPixels(info, data->writable_data(), mStride, 0, 0, SkImage::kDisallow_CachingHint)) {
           raster = SkImage::MakeRasterData(info, data, mStride);
         }
       }
     }
-    if (!raster) {
+    if (raster) {
+      mImage = raster;
+    } else {
       gfxCriticalError() << "Failed making Skia raster image for GPU surface";
     }
-    mImage = raster;
   }
 #endif
   SkPixmap pixmap;
   if (!mImage->peekPixels(&pixmap)) {
     gfxCriticalError() << "Failed accessing pixels for Skia raster image";
   }
   return reinterpret_cast<uint8_t*>(pixmap.writable_addr());
 }

--- a/image/ScriptedNotificationObserver.cpp
+++ b/image/ScriptedNotificationObserver.cpp
@@ -26,16 +26,19 @@ ScriptedNotificationObserver::ScriptedNo
 : mInner(aInner)
 { }
 
 NS_IMETHODIMP
 ScriptedNotificationObserver::Notify(imgIRequest* aRequest,
                                      int32_t aType,
                                      const nsIntRect* /*aUnused*/)
 {
+  MOZ_RELEASE_ASSERT(js::AllowGCBarriers(CycleCollectedJSContext::Get()->Context()),
+                     "sending image notification to JS observer during painting. See bug 1311841");
+
   if (aType == imgINotificationObserver::SIZE_AVAILABLE) {
     return mInner->SizeAvailable(aRequest);
   }
   if (aType == imgINotificationObserver::FRAME_UPDATE) {
     return mInner->FrameUpdate(aRequest);
   }
   if (aType == imgINotificationObserver::FRAME_COMPLETE) {
     return mInner->FrameComplete(aRequest);

--- a/js/src/jit/Ion.cpp
+++ b/js/src/jit/Ion.cpp
@@ -1517,16 +1517,27 @@ OptimizeMIR(MIRGenerator* mir)
         gs.spewPass("Prune Unused Branches");
         AssertBasicGraphCoherency(graph);
 
         if (mir->shouldCancel("Prune Unused Branches"))
             return false;
     }
 
     {
+        AutoTraceLog log(logger, TraceLogger_FoldEmptyBlocks);
+        if (!FoldEmptyBlocks(graph))
+            return false;
+        gs.spewPass("Fold Empty Blocks");
+        AssertBasicGraphCoherency(graph);
+
+        if (mir->shouldCancel("Fold Empty Blocks"))
+            return false;
+    }
+
+    {
         AutoTraceLog log(logger, TraceLogger_FoldTests);
         if (!FoldTests(graph))
             return false;
         gs.spewPass("Fold Tests");
         AssertBasicGraphCoherency(graph);
 
         if (mir->shouldCancel("Fold Tests"))
             return false;

--- a/js/src/jit/IonAnalysis.cpp
+++ b/js/src/jit/IonAnalysis.cpp
@@ -918,16 +918,52 @@ jit::FoldTests(MIRGraph& graph)
 {
     for (MBasicBlockIterator block(graph.begin()); block != graph.end(); block++) {
         if (!MaybeFoldConditionBlock(graph, *block))
             return false;
     }
     return true;
 }
 
+bool
+jit::FoldEmptyBlocks(MIRGraph& graph)
+{
+    for (MBasicBlockIterator iter(graph.begin()); iter != graph.end(); ) {
+        MBasicBlock* block = *iter;
+        iter++;
+
+        if (block->numPredecessors() != 1 || block->numSuccessors() != 1)
+            continue;
+
+        if (!block->phisEmpty())
+            continue;
+
+        if (block->outerResumePoint())
+            continue;
+
+        if (*block->begin() != *block->rbegin())
+            continue;
+
+        MBasicBlock* succ = block->getSuccessor(0);
+        MBasicBlock* pred = block->getPredecessor(0);
+
+        if (succ->numPredecessors() != 1)
+            continue;
+
+        size_t pos = pred->getSuccessorIndex(block);
+        pred->lastIns()->replaceSuccessor(pos, succ);
+
+        graph.removeBlock(block);
+
+        succ->addPredecessorSameInputsAs(pred, block);
+        succ->removePredecessor(block);
+    }
+    return true;
+}
+
 static void
 EliminateTriviallyDeadResumePointOperands(MIRGraph& graph, MResumePoint* rp)
 {
     // If we will pop the top of the stack immediately after resuming,
     // then don't preserve the top value in the resume point.
     if (rp->mode() != MResumePoint::ResumeAt || *rp->pc() != JSOP_POP)
         return;
 

--- a/js/src/jit/IonAnalysis.h
+++ b/js/src/jit/IonAnalysis.h
@@ -20,16 +20,19 @@ class MIRGraph;
 
 MOZ_MUST_USE bool
 PruneUnusedBranches(MIRGenerator* mir, MIRGraph& graph);
 
 MOZ_MUST_USE bool
 FoldTests(MIRGraph& graph);
 
 MOZ_MUST_USE bool
+FoldEmptyBlocks(MIRGraph& graph);
+
+MOZ_MUST_USE bool
 SplitCriticalEdges(MIRGraph& graph);
 
 bool
 IsUint32Type(const MDefinition* def);
 
 enum Observability {
     ConservativeObservability,
     AggressiveObservability

--- a/js/src/vm/TraceLoggingTypes.h
+++ b/js/src/vm/TraceLoggingTypes.h
@@ -37,16 +37,17 @@
     _(VM)                                             \
     _(CompressSource)                                 \
     _(WasmCompilation)                                \
     _(Call)                                           \
                                                       \
     /* Specific passes during ion compilation */      \
     _(PruneUnusedBranches)                            \
     _(FoldTests)                                      \
+    _(FoldEmptyBlocks)                                \
     _(SplitCriticalEdges)                             \
     _(RenumberBlocks)                                 \
     _(ScalarReplacement)                              \
     _(DominatorTree)                                  \
     _(PhiAnalysis)                                    \
     _(MakeLoopsContiguous)                            \
     _(ApplyTypes)                                     \
     _(EagerSimdUnbox)                                 \

--- a/layout/base/nsLayoutUtils.cpp
+++ b/layout/base/nsLayoutUtils.cpp
@@ -79,16 +79,17 @@
 #include "nsCSSProps.h"
 #include "nsListControlFrame.h"
 #include "mozilla/dom/Element.h"
 #include "nsCanvasFrame.h"
 #include "gfxDrawable.h"
 #include "gfxEnv.h"
 #include "gfxUtils.h"
 #include "nsDataHashtable.h"
+#include "nsTableWrapperFrame.h"
 #include "nsTextFrame.h"
 #include "nsFontFaceList.h"
 #include "nsFontInflationData.h"
 #include "nsSVGUtils.h"
 #include "SVGImageContext.h"
 #include "SVGTextFrame.h"
 #include "nsStyleStructInlines.h"
 #include "nsStyleTransformMatrix.h"
@@ -5863,16 +5864,29 @@ nsLayoutUtils::GetFirstLinePosition(Writ
   const nsBlockFrame* block = nsLayoutUtils::GetAsBlock(const_cast<nsIFrame*>(aFrame));
   if (!block) {
     // For the first-line baseline we also have to check for a table, and if
     // so, use the baseline of its first row.
     nsIAtom* fType = aFrame->GetType();
     if (fType == nsGkAtoms::tableWrapperFrame  ||
         fType == nsGkAtoms::flexContainerFrame ||
         fType == nsGkAtoms::gridContainerFrame) {
+      if ((fType == nsGkAtoms::gridContainerFrame &&
+           aFrame->HasAnyStateBits(NS_STATE_GRID_SYNTHESIZE_BASELINE)) ||
+          (fType == nsGkAtoms::flexContainerFrame &&
+           aFrame->HasAnyStateBits(NS_STATE_FLEX_SYNTHESIZE_BASELINE)) ||
+          (fType == nsGkAtoms::tableWrapperFrame &&
+           static_cast<const nsTableWrapperFrame*>(aFrame)->GetRowCount() == 0)) {
+        // empty grid/flex/table container
+        aResult->mBStart = 0;
+        aResult->mBaseline = aFrame->SynthesizeBaselineBOffsetFromBorderBox(aWM,
+                                       BaselineSharingGroup::eFirst);
+        aResult->mBEnd = aFrame->BSize(aWM);
+        return true;
+      }
       aResult->mBStart = 0;
       aResult->mBaseline = aFrame->GetLogicalBaseline(aWM);
       // This is what we want for the list bullet caller; not sure if
       // other future callers will want the same.
       aResult->mBEnd = aFrame->BSize(aWM);
       return true;
     }
 

--- a/layout/generic/nsBlockFrame.cpp
+++ b/layout/generic/nsBlockFrame.cpp
@@ -485,22 +485,54 @@ nsBlockFrame::InvalidateFrameWithRect(co
                  "unexpected block frame in SVG text");
     GetParent()->InvalidateFrame();
     return;
   }
   nsContainerFrame::InvalidateFrameWithRect(aRect, aDisplayItemKey);
 }
 
 nscoord
-nsBlockFrame::GetLogicalBaseline(WritingMode aWritingMode) const
-{
-  nscoord result;
-  if (nsLayoutUtils::GetLastLineBaseline(aWritingMode, this, &result))
-    return result;
-  return nsFrame::GetLogicalBaseline(aWritingMode);
+nsBlockFrame::GetLogicalBaseline(WritingMode aWM) const
+{
+  auto lastBaseline =
+    BaselineBOffset(aWM, BaselineSharingGroup::eLast, AlignmentContext::eInline);
+  return BSize(aWM) - lastBaseline;
+}
+
+bool
+nsBlockFrame::GetNaturalBaselineBOffset(mozilla::WritingMode aWM,
+                                        BaselineSharingGroup aBaselineGroup,
+                                        nscoord*             aBaseline) const
+{
+  if (aBaselineGroup == BaselineSharingGroup::eFirst) {
+    return nsLayoutUtils::GetFirstLineBaseline(aWM, this, aBaseline);
+  }
+
+  for (ConstReverseLineIterator line = LinesRBegin(), line_end = LinesREnd();
+       line != line_end; ++line) {
+    if (line->IsBlock()) {
+      nscoord offset;
+      nsIFrame* kid = line->mFirstChild;
+      if (kid->GetVerticalAlignBaseline(aWM, &offset)) {
+        // Ignore relative positioning for baseline calculations.
+        const nsSize& sz = line->mContainerSize;
+        offset += kid->GetLogicalNormalPosition(aWM, sz).B(aWM);
+        *aBaseline = BSize(aWM) - offset;
+        return true;
+      }
+    } else {
+      // XXX Is this the right test?  We have some bogus empty lines
+      // floating around, but IsEmpty is perhaps too weak.
+      if (line->BSize() != 0 || !line->IsEmpty()) {
+        *aBaseline = BSize(aWM) - (line->BStart() + line->GetLogicalAscent());
+        return true;
+      }
+    }
+  }
+  return false;
 }
 
 nscoord
 nsBlockFrame::GetCaretBaseline() const
 {
   nsRect contentRect = GetContentRect();
   nsMargin bp = GetUsedBorderAndPadding();
 

--- a/layout/generic/nsBlockFrame.h
+++ b/layout/generic/nsBlockFrame.h
@@ -116,16 +116,29 @@ public:
   virtual void InsertFrames(ChildListID aListID,
                             nsIFrame* aPrevFrame,
                             nsFrameList& aFrameList) override;
   virtual void RemoveFrame(ChildListID aListID,
                            nsIFrame* aOldFrame) override;
   virtual const nsFrameList& GetChildList(ChildListID aListID) const override;
   virtual void GetChildLists(nsTArray<ChildList>* aLists) const override;
   virtual nscoord GetLogicalBaseline(mozilla::WritingMode aWritingMode) const override;
+  bool GetVerticalAlignBaseline(mozilla::WritingMode aWM,
+                                nscoord* aBaseline) const override
+  {
+    nscoord lastBaseline;
+    if (GetNaturalBaselineBOffset(aWM, BaselineSharingGroup::eLast, &lastBaseline)) {
+      *aBaseline = BSize() - lastBaseline;
+      return true;
+    }
+    return false;
+  }
+  bool GetNaturalBaselineBOffset(mozilla::WritingMode aWM,
+                                 BaselineSharingGroup aBaselineGroup,
+                                 nscoord*             aBaseline) const override;
   virtual nscoord GetCaretBaseline() const override;
   virtual void DestroyFrom(nsIFrame* aDestructRoot) override;
   virtual nsSplittableType GetSplittableType() const override;
   virtual bool IsFloatContainingBlock() const override;
   virtual void BuildDisplayList(nsDisplayListBuilder* aBuilder,
                                 const nsRect& aDirtyRect,
                                 const nsDisplayListSet& aLists) override;
   virtual nsIAtom* GetType() const override;

--- a/layout/generic/nsContainerFrame.cpp
+++ b/layout/generic/nsContainerFrame.cpp
@@ -1016,16 +1016,17 @@ nsContainerFrame::ReflowChild(nsIFrame* 
 
   // Position the child frame and its view if requested.
   if (NS_FRAME_NO_MOVE_FRAME != (aFlags & NS_FRAME_NO_MOVE_FRAME)) {
     aKidFrame->SetPosition(aWM, aPos, aContainerSize);
   }
 
   if (0 == (aFlags & NS_FRAME_NO_MOVE_VIEW)) {
     PositionFrameView(aKidFrame);
+    PositionChildViews(aKidFrame);
   }
 
   // Reflow the child frame
   aKidFrame->Reflow(aPresContext, aDesiredSize, aReflowInput, aStatus);
 
   // If the child frame is complete, delete any next-in-flows,
   // but only if the NO_DELETE_NEXT_IN_FLOW flag isn't set.
   if (!NS_INLINE_IS_BREAK_BEFORE(aStatus) &&
@@ -1059,16 +1060,17 @@ nsContainerFrame::ReflowChild(nsIFrame* 
 
   // Position the child frame and its view if requested.
   if (NS_FRAME_NO_MOVE_FRAME != (aFlags & NS_FRAME_NO_MOVE_FRAME)) {
     aKidFrame->SetPosition(nsPoint(aX, aY));
   }
 
   if (0 == (aFlags & NS_FRAME_NO_MOVE_VIEW)) {
     PositionFrameView(aKidFrame);
+    PositionChildViews(aKidFrame);
   }
 
   // Reflow the child frame
   aKidFrame->Reflow(aPresContext, aDesiredSize, aReflowInput, aStatus);
 
   // If the child frame is complete, delete any next-in-flows,
   // but only if the NO_DELETE_NEXT_IN_FLOW flag isn't set.
   if (NS_FRAME_IS_FULLY_COMPLETE(aStatus) &&

--- a/layout/generic/nsFlexContainerFrame.cpp
+++ b/layout/generic/nsFlexContainerFrame.cpp
@@ -450,17 +450,18 @@ public:
       // or a dependency.
       // Use GetFirstLineBaseline() or GetLastLineBaseline() as appropriate,
       // or just GetLogicalBaseline() if that fails.
       bool found = aUseFirstBaseline ?
         nsLayoutUtils::GetFirstLineBaseline(mWM, mFrame, &mAscent) :
         nsLayoutUtils::GetLastLineBaseline(mWM, mFrame, &mAscent);
 
       if (!found) {
-        mAscent = mFrame->GetLogicalBaseline(mWM);
+        mAscent = mFrame->SynthesizeBaselineBOffsetFromBorderBox(mWM,
+                            BaselineSharingGroup::eFirst);
       }
     }
     return mAscent;
   }
 
   // Convenience methods to compute the main & cross size of our *margin-box*.
   // The caller is responsible for telling us the right axis, so that we can
   // pull out the appropriate components of our margin/border/padding structs.
@@ -2319,16 +2320,20 @@ nsFlexContainerFrame::GetFrameName(nsASt
 #endif
 
 nscoord
 nsFlexContainerFrame::GetLogicalBaseline(mozilla::WritingMode aWM) const
 {
   NS_ASSERTION(mBaselineFromLastReflow != NS_INTRINSIC_WIDTH_UNKNOWN,
                "baseline has not been set");
 
+  if (HasAnyStateBits(NS_STATE_FLEX_SYNTHESIZE_BASELINE)) {
+    // Return a baseline synthesized from our margin-box.
+    return nsContainerFrame::GetLogicalBaseline(aWM);
+  }
   return mBaselineFromLastReflow;
 }
 
 // Helper for BuildDisplayList, to implement this special-case for flex items
 // from the spec:
 //    Flex items paint exactly the same as block-level elements in the
 //    normal flow, except that 'z-index' values other than 'auto' create
 //    a stacking context even if 'position' is 'static'.
@@ -4254,16 +4259,24 @@ nsFlexContainerFrame::DoFlexLayout(nsPre
   AutoFlexLineListClearer cleanupLines(lines);
 
   GenerateFlexLines(aPresContext, aReflowInput,
                     aContentBoxMainSize,
                     aAvailableBSizeForContent,
                     aStruts, aAxisTracker,
                     placeholderKids, lines);
 
+  if (lines.getFirst()->IsEmpty() &&
+      !lines.getFirst()->getNext()) {
+    // We have no flex items, our parent should synthesize a baseline if needed.
+    AddStateBits(NS_STATE_FLEX_SYNTHESIZE_BASELINE);
+  } else {
+    RemoveStateBits(NS_STATE_FLEX_SYNTHESIZE_BASELINE);
+  }
+
   aContentBoxMainSize =
     ResolveFlexContainerMainSize(aReflowInput, aAxisTracker,
                                  aContentBoxMainSize, aAvailableBSizeForContent,
                                  lines.getFirst(), aStatus);
 
   for (FlexLine* line = lines.getFirst(); line; line = line->getNext()) {
     line->ResolveFlexibleLengths(aContentBoxMainSize);
   }
@@ -4514,22 +4527,25 @@ nsFlexContainerFrame::DoFlexLayout(nsPre
     // Per spec, synthesize baseline from the flex container's content box
     // (i.e. use block-end side of content-box)
     // XXXdholbert This only makes sense if parent's writing mode is
     // horizontal (& even then, really we should be using the BSize in terms
     // of the parent's writing mode, not ours). Clean up in bug 1155322.
     flexContainerAscent = desiredSizeInFlexWM.BSize(flexWM);
   }
 
-  // XXXdholbert flexContainerAscent needs to be in terms of
-  // our parent's writing-mode here. See bug 1155322.
-  aDesiredSize.SetBlockStartAscent(flexContainerAscent);
-
-  // Cache this baseline for use outside of this call.
-  mBaselineFromLastReflow = flexContainerAscent;
+  if (HasAnyStateBits(NS_STATE_FLEX_SYNTHESIZE_BASELINE)) {
+    // This will force our parent to call GetLogicalBaseline, which will
+    // synthesize a margin-box baseline.
+    aDesiredSize.SetBlockStartAscent(ReflowOutput::ASK_FOR_BASELINE);
+  } else {
+    // XXXdholbert flexContainerAscent needs to be in terms of
+    // our parent's writing-mode here. See bug 1155322.
+    aDesiredSize.SetBlockStartAscent(flexContainerAscent);
+  }
 
   // Now: If we're complete, add bottom border/padding to desired height (which
   // we skipped via skipSides) -- unless that pushes us over available height,
   // in which case we become incomplete (unless we already weren't asking for
   // any height, in which case we stay complete to avoid looping forever).
   // NOTE: If we're auto-height, we allow our bottom border/padding to push us
   // over the available height without requesting a continuation, for
   // consistency with the behavior of "display:block" elements.
@@ -4544,16 +4560,26 @@ nsFlexContainerFrame::DoFlexLayout(nsPre
       // Update desired height to include block-end border/padding
       desiredSizeInFlexWM.BSize(flexWM) = desiredBSizeWithBEndBP;
     } else {
       // We couldn't fit bottom border/padding, so we'll need a continuation.
       NS_FRAME_SET_INCOMPLETE(aStatus);
     }
   }
 
+  // Calculate the container baselines so that our parent can baseline-align us.
+  mBaselineFromLastReflow = flexContainerAscent;
+  mLastBaselineFromLastReflow = lines.getLast()->GetLastBaselineOffset();
+  if (mLastBaselineFromLastReflow == nscoord_MIN) {
+    // XXX we fall back to a mirrored first baseline here for now, but this
+    // should probably use the last baseline of the last item or something.
+    mLastBaselineFromLastReflow =
+      desiredSizeInFlexWM.BSize(flexWM) - flexContainerAscent;
+  }
+
   // Convert flex container's final desired size to parent's WM, for outparam.
   aDesiredSize.SetSize(flexWM, desiredSizeInFlexWM);
 
   // Overflow area = union(my overflow area, kids' overflow areas)
   aDesiredSize.SetOverflowAreasToDesiredBounds();
   for (nsIFrame* childFrame : mFrames) {
     ConsiderChildOverflow(aDesiredSize.mOverflowAreas, childFrame);
   }

--- a/layout/generic/nsFlexContainerFrame.h
+++ b/layout/generic/nsFlexContainerFrame.h
@@ -78,16 +78,34 @@ public:
 
   virtual nsIAtom* GetType() const override;
 #ifdef DEBUG_FRAME_DUMP
   virtual nsresult GetFrameName(nsAString& aResult) const override;
 #endif
 
   nscoord GetLogicalBaseline(mozilla::WritingMode aWM) const override;
 
+  bool GetVerticalAlignBaseline(mozilla::WritingMode aWM,
+                                nscoord* aBaseline) const override
+  {
+    return GetNaturalBaselineBOffset(aWM, BaselineSharingGroup::eFirst, aBaseline);
+  }
+
+  bool GetNaturalBaselineBOffset(mozilla::WritingMode aWM,
+                                 BaselineSharingGroup aBaselineGroup,
+                                 nscoord*             aBaseline) const override
+  {
+    if (HasAnyStateBits(NS_STATE_FLEX_SYNTHESIZE_BASELINE)) {
+      return false;
+    }
+    *aBaseline = aBaselineGroup == BaselineSharingGroup::eFirst ?
+                   mBaselineFromLastReflow : mLastBaselineFromLastReflow;
+    return true;
+  }
+
   // nsContainerFrame overrides
   uint16_t CSSAlignmentForAbsPosChild(
             const ReflowInput& aChildRI,
             mozilla::LogicalAxis aLogicalAxis) const override;
 
   // Flexbox-specific public methods
   bool IsHorizontal();
 
@@ -111,16 +129,17 @@ public:
                                     uint32_t* aNumPackingSpacesRemaining,
                                     nscoord* aPackingSpaceRemaining);
 
 protected:
   // Protected constructor & destructor
   explicit nsFlexContainerFrame(nsStyleContext* aContext)
     : nsContainerFrame(aContext)
     , mBaselineFromLastReflow(NS_INTRINSIC_WIDTH_UNKNOWN)
+    , mLastBaselineFromLastReflow(NS_INTRINSIC_WIDTH_UNKNOWN)
   {}
   virtual ~nsFlexContainerFrame();
 
   /*
    * This method does the bulk of the flex layout, implementing the algorithm
    * described at:
    *   http://dev.w3.org/csswg/css-flexbox/#layout-algorithm
    * (with a few initialization pieces happening in the caller, Reflow().
@@ -296,11 +315,13 @@ protected:
                           nsTArray<nsIFrame*>& aPlaceholders,
                           const mozilla::LogicalPoint& aContentBoxOrigin,
                           const nsSize& aContainerSize);
 
   bool mChildrenHaveBeenReordered; // Have we ever had to reorder our kids
                                    // to satisfy their 'order' values?
 
   nscoord mBaselineFromLastReflow;
+  // Note: the last baseline is a distance from our border-box end edge.
+  nscoord mLastBaselineFromLastReflow;
 };
 
 #endif /* nsFlexContainerFrame_h___ */

--- a/layout/generic/nsFrameStateBits.h
+++ b/layout/generic/nsFrameStateBits.h
@@ -306,16 +306,19 @@ FRAME_STATE_GROUP(FlexContainer, nsFlexC
 // Set for a flex container whose children have been reordered due to 'order'.
 // (Means that we have to be more thorough about checking them for sortedness.)
 FRAME_STATE_BIT(FlexContainer, 20, NS_STATE_FLEX_CHILDREN_REORDERED)
 
 // Set for a flex container that is emulating a legacy
 // 'display:-webkit-{inline-}box' container.
 FRAME_STATE_BIT(FlexContainer, 21, NS_STATE_FLEX_IS_LEGACY_WEBKIT_BOX)
 
+// True if the container has no flex items; may lie if there is a pending reflow
+FRAME_STATE_BIT(FlexContainer, 22, NS_STATE_FLEX_SYNTHESIZE_BASELINE)
+
 // == Frame state bits that apply to grid container frames ====================
 
 FRAME_STATE_GROUP(GridContainer, nsGridContainerFrame)
 
 // True iff the normal flow children are already in CSS 'order' in the
 // order they occur in the child frame list.
 FRAME_STATE_BIT(GridContainer, 20, NS_STATE_GRID_NORMAL_FLOW_CHILDREN_IN_CSS_ORDER)
 
@@ -323,16 +326,19 @@ FRAME_STATE_BIT(GridContainer, 20, NS_ST
 // Note that those child frames may have been removed without this bit
 // being updated for performance reasons, so code shouldn't depend on
 // actually finding any pushed items when this bit is set.
 FRAME_STATE_BIT(GridContainer, 21, NS_STATE_GRID_DID_PUSH_ITEMS)
 
 // True iff computed grid values should be generated on the next reflow
 FRAME_STATE_BIT(GridContainer, 22, NS_STATE_GRID_GENERATE_COMPUTED_VALUES)
 
+// True if the container has no grid items; may lie if there is a pending reflow
+FRAME_STATE_BIT(GridContainer, 23, NS_STATE_GRID_SYNTHESIZE_BASELINE)
+
 // == Frame state bits that apply to SVG frames ===============================
 
 FRAME_STATE_GROUP(SVG, nsISVGChildFrame)
 FRAME_STATE_GROUP(SVG, nsSVGContainerFrame)
 
 FRAME_STATE_BIT(SVG, 20, NS_STATE_IS_OUTER_SVG)
 
 // If this bit is set, we are a <clipPath> element or descendant.

--- a/layout/generic/nsGfxScrollFrame.cpp
+++ b/layout/generic/nsGfxScrollFrame.cpp
@@ -6154,8 +6154,65 @@ ScrollFrameHelper::GetSnapPointForDestin
 bool
 ScrollFrameHelper::UsesContainerScrolling() const
 {
   if (gfxPrefs::LayoutUseContainersForRootFrames()) {
     return mIsRoot;
   }
   return false;
 }
+
+bool
+ScrollFrameHelper::DragScroll(WidgetEvent* aEvent)
+{
+  // Dragging is allowed while within a 20 pixel border. Note that device pixels
+  // are used so that the same margin is used even when zoomed in or out.
+  nscoord margin = 20 * mOuter->PresContext()->AppUnitsPerDevPixel();
+
+  // Don't drag scroll for small scrollareas.
+  if (mScrollPort.width < margin * 2 || mScrollPort.height < margin * 2) {
+    return false;
+  }
+
+  // If willScroll is computed as false, then the frame is already scrolled as
+  // far as it can go in both directions. Return false so that an ancestor
+  // scrollframe can scroll instead.
+  bool willScroll = false;
+  nsPoint pnt = nsLayoutUtils::GetEventCoordinatesRelativeTo(aEvent, mOuter);
+  nsPoint scrollPoint = GetScrollPosition();
+  nsRect rangeRect = GetScrollRangeForClamping();
+
+  // Only drag scroll when a scrollbar is present.
+  nsPoint offset;
+  if (mHasHorizontalScrollbar) {
+    if (pnt.x >= mScrollPort.x && pnt.x <= mScrollPort.x + margin) {
+      offset.x = -margin;
+      if (scrollPoint.x > 0) {
+        willScroll = true;
+      }
+    } else if (pnt.x >= mScrollPort.XMost() - margin && pnt.x <= mScrollPort.XMost()) {
+      offset.x = margin;
+      if (scrollPoint.x < rangeRect.width) {
+        willScroll = true;
+      }
+    }
+  }
+
+  if (mHasVerticalScrollbar) {
+    if (pnt.y >= mScrollPort.y && pnt.y <= mScrollPort.y + margin) {
+      offset.y = -margin;
+      if (scrollPoint.y > 0) {
+        willScroll = true;
+      }
+    } else if (pnt.y >= mScrollPort.YMost() - margin && pnt.y <= mScrollPort.YMost()) {
+      offset.y = margin;
+      if (scrollPoint.y < rangeRect.height) {
+        willScroll = true;
+      }
+    }
+  }
+
+  if (offset.x || offset.y) {
+    ScrollTo(GetScrollPosition() + offset, nsIScrollableFrame::NORMAL);
+  }
+
+  return willScroll;
+}

--- a/layout/generic/nsGfxScrollFrame.h
+++ b/layout/generic/nsGfxScrollFrame.h
@@ -455,16 +455,18 @@ public:
                     int32_t aDirection,
                     nsIScrollableFrame::ScrollUnit aUnit,
                     nsIScrollbarMediator::ScrollSnapMode aSnap
                       = nsIScrollbarMediator::DISABLE_SNAP);
   bool ShouldSuppressScrollbarRepaints() const {
     return mSuppressScrollbarRepaints;
   }
 
+  bool DragScroll(WidgetEvent* aEvent);
+
   // owning references to the nsIAnonymousContentCreator-built content
   nsCOMPtr<nsIContent> mHScrollbarContent;
   nsCOMPtr<nsIContent> mVScrollbarContent;
   nsCOMPtr<nsIContent> mScrollCornerContent;
   nsCOMPtr<nsIContent> mResizerContent;
 
   RefPtr<ScrollEvent> mScrollEvent;
   nsRevocableEventPtr<AsyncScrollPortEvent> mAsyncScrollPortEvent;
@@ -719,16 +721,22 @@ public:
                       ReflowOutput&     aDesiredSize,
                       const ReflowInput& aReflowInput,
                       nsReflowStatus&          aStatus) override;
 
   virtual bool ComputeCustomOverflow(nsOverflowAreas& aOverflowAreas) override {
     return mHelper.ComputeCustomOverflow(aOverflowAreas);
   }
 
+  bool GetVerticalAlignBaseline(mozilla::WritingMode aWM,
+                                nscoord* aBaseline) const override {
+    *aBaseline = GetLogicalBaseline(aWM);
+    return true;
+  }
+
   // Recomputes the scrollable overflow area we store in the helper to take children
   // that are affected by perpsective set on the outer frame and scroll at different
   // rates.
   void AdjustForPerspective(nsRect& aScrollableOverflow);
 
   // Called to set the child frames. We typically have three: the scroll area,
   // the vertical scrollbar, and the horizontal scrollbar.
   virtual void SetInitialChildList(ChildListID     aListID,
@@ -1028,16 +1036,20 @@ public:
   void SetScrollsClipOnUnscrolledOutOfFlow() override {
     mHelper.SetScrollsClipOnUnscrolledOutOfFlow();
   }
   
   ScrollSnapInfo GetScrollSnapInfo() const override {
     return mHelper.GetScrollSnapInfo();
   }
 
+  virtual bool DragScroll(mozilla::WidgetEvent* aEvent) override {
+    return mHelper.DragScroll(aEvent);
+  }
+
 #ifdef DEBUG_FRAME_DUMP
   virtual nsresult GetFrameName(nsAString& aResult) const override;
 #endif
 
 #ifdef ACCESSIBILITY
   virtual mozilla::a11y::AccType AccessibleType() override;
 #endif
 
@@ -1105,16 +1117,22 @@ public:
 #if 0
   virtual nscoord GetMinISize(nsRenderingContext *aRenderingContext) override;
 #endif
 
   virtual bool ComputeCustomOverflow(nsOverflowAreas& aOverflowAreas) override {
     return mHelper.ComputeCustomOverflow(aOverflowAreas);
   }
 
+  bool GetVerticalAlignBaseline(mozilla::WritingMode aWM,
+                                nscoord* aBaseline) const override {
+    *aBaseline = GetLogicalBaseline(aWM);
+    return true;
+  }
+
   // Called to set the child frames. We typically have three: the scroll area,
   // the vertical scrollbar, and the horizontal scrollbar.
   virtual void SetInitialChildList(ChildListID     aListID,
                                    nsFrameList&    aChildList) override;
   virtual void AppendFrames(ChildListID     aListID,
                             nsFrameList&    aFrameList) override;
   virtual void InsertFrames(ChildListID     aListID,
                             nsIFrame*       aPrevFrame,
@@ -1451,16 +1469,20 @@ public:
   void TriggerDisplayPortExpiration() override {
     mHelper.TriggerDisplayPortExpiration();
   }
 
   ScrollSnapInfo GetScrollSnapInfo() const override {
     return mHelper.GetScrollSnapInfo();
   }
 
+  virtual bool DragScroll(mozilla::WidgetEvent* aEvent) override {
+    return mHelper.DragScroll(aEvent);
+  }
+
 #ifdef DEBUG_FRAME_DUMP
   virtual nsresult GetFrameName(nsAString& aResult) const override;
 #endif
 
 protected:
   nsXULScrollFrame(nsStyleContext* aContext, bool aIsRoot,
                    bool aClipAllDescendants);
 

--- a/layout/generic/nsGridContainerFrame.cpp
+++ b/layout/generic/nsGridContainerFrame.cpp
@@ -193,17 +193,17 @@ enum class GridLineSide
 struct nsGridContainerFrame::TrackSize
 {
   enum StateBits : uint16_t {
     eAutoMinSizing =              0x1,
     eMinContentMinSizing =        0x2,
     eMaxContentMinSizing =        0x4,
     eMinOrMaxContentMinSizing = eMinContentMinSizing | eMaxContentMinSizing,
     eIntrinsicMinSizing = eMinOrMaxContentMinSizing | eAutoMinSizing,
-    eIndefinitePercentMinSizing = 0x8,
+    // 0x8 is unused, feel free to take it!
     eAutoMaxSizing =             0x10,
     eMinContentMaxSizing =       0x20,
     eMaxContentMaxSizing =       0x40,
     eAutoOrMaxContentMaxSizing = eAutoMaxSizing | eMaxContentMaxSizing,
     eIntrinsicMaxSizing = eAutoOrMaxContentMaxSizing | eMinContentMaxSizing,
     eFlexMaxSizing =             0x80,
     eFrozen =                   0x100,
     eSkipGrowUnlimited1 =       0x200,
@@ -264,17 +264,16 @@ nsGridContainerFrame::TrackSize::Initial
     minSizeUnit = eStyleUnit_Auto;
     maxSizeUnit = eStyleUnit_Enumerated; // triggers max-content sizing below
   }
   if (::IsPercentOfIndefiniteSize(aMinCoord, aPercentageBasis)) {
     // https://drafts.csswg.org/css-grid/#valdef-grid-template-columns-percentage
     // "If the inline or block size of the grid container is indefinite,
     //  <percentage> values relative to that size are treated as 'auto'."
     minSizeUnit = eStyleUnit_Auto;
-    mState |= eIndefinitePercentMinSizing;
   }
   if (::IsPercentOfIndefiniteSize(aMaxCoord, aPercentageBasis)) {
     maxSizeUnit = eStyleUnit_Auto;
   }
   // http://dev.w3.org/csswg/css-grid/#algo-init
   switch (minSizeUnit) {
     case eStyleUnit_Auto:
       mState |= eAutoMinSizing;
@@ -1157,69 +1156,64 @@ struct nsGridContainerFrame::TrackSizing
     const uint32_t numTracks = mMinSizingFunctions.Length();
     MOZ_ASSERT(numTracks >= 1, "expected at least the repeat() track");
     nscoord maxFill = aSize != NS_UNCONSTRAINEDSIZE ? aSize : aMaxSize;
     if (maxFill == NS_UNCONSTRAINEDSIZE && aMinSize == 0) {
       // "Otherwise, the specified track list repeats only once."
       return 1;
     }
     nscoord repeatTrackSize = 0;
-    float repeatTrackPercent = 0.0f;
     // Note that the repeat() track size is included in |sum| in this loop.
     nscoord sum = 0;
-    float percentSum = 0.0f;
     const nscoord percentBasis = aSize;
     for (uint32_t i = 0; i < numTracks; ++i) {
       // "treating each track as its max track sizing function if that is
       // definite or as its minimum track sizing function otherwise"
       // https://drafts.csswg.org/css-grid/#valdef-repeat-auto-fill
       const auto& maxCoord = mMaxSizingFunctions[i];
       const auto* coord = &maxCoord;
       if (!coord->IsCoordPercentCalcUnit()) {
         coord = &mMinSizingFunctions[i];
         if (!coord->IsCoordPercentCalcUnit()) {
           return 1;
         }
       }
-      float trackPercent;
-      nscoord trackSize;
-      ResolvePercentSizeParts(*coord, percentBasis, &trackSize, &trackPercent);
+      nscoord trackSize = ::ResolveToDefiniteSize(*coord, percentBasis);
       if (i == mRepeatAutoStart) {
         if (percentBasis != NS_UNCONSTRAINEDSIZE) {
           // Use a minimum 1px for the repeat() track-size.
           if (trackSize < AppUnitsPerCSSPixel()) {
             trackSize = AppUnitsPerCSSPixel();
           }
         }
         repeatTrackSize = trackSize;
-        repeatTrackPercent = trackPercent;
       }
       sum += trackSize;
-      percentSum += trackPercent;
     }
     nscoord gridGap;
+    float percentSum = 0.0f;
     float gridGapPercent;
     ResolvePercentSizeParts(aGridGap, percentBasis, &gridGap, &gridGapPercent);
     if (numTracks > 1) {
       // Add grid-gaps for all the tracks including the repeat() track.
       sum += gridGap * (numTracks - 1);
-      percentSum += gridGapPercent * (numTracks - 1);
+      percentSum = gridGapPercent * (numTracks - 1);
     }
     // Calculate the max number of tracks that fits without overflow.
     nscoord available = maxFill != NS_UNCONSTRAINEDSIZE ? maxFill : aMinSize;
     nscoord size = nsLayoutUtils::AddPercents(sum, percentSum);
     if (available - size < 0) {
       // "if any number of repetitions would overflow, then 1 repetition"
       return 1;
     }
     uint32_t numRepeatTracks = 1;
     bool exactFit = false;
     while (true) {
       sum += gridGap + repeatTrackSize;
-      percentSum += gridGapPercent + repeatTrackPercent;
+      percentSum += gridGapPercent;
       nscoord newSize = nsLayoutUtils::AddPercents(sum, percentSum);
       if (newSize <= size) {
         // Adding more repeat-tracks won't make forward progress.
         return numRepeatTracks;
       }
       size = newSize;
       nscoord remaining = available - size;
       exactFit = remaining == 0;
@@ -2582,24 +2576,18 @@ nsGridContainerFrame::GridReflowInput::C
   mCols.mCanResolveLineRangeSize = true;
 
   mRows.CalculateSizes(*this, mGridItems, mRowFunctions,
                        aContentBox.BSize(mWM), &GridArea::mRows,
                        aConstraint);
   if (aContentBox.BSize(mWM) == NS_AUTOHEIGHT) {
     aContentBox.BSize(mWM) =
       mRows.BackComputedIntrinsicSize(mRowFunctions, mGridStyle->mGridRowGap);
-    if ((mRows.mStateUnion & TrackSize::eIndefinitePercentMinSizing) ||
-        mGridStyle->mGridRowGap.HasPercent()) {
-      mRows.Initialize(mRowFunctions, mGridStyle->mGridRowGap,
-                       aGrid.mGridRowEnd, aContentBox.BSize(mWM));
-      mRows.CalculateSizes(*this, mGridItems, mRowFunctions,
-                           aContentBox.BSize(mWM), &GridArea::mRows,
-                           aConstraint);
-    }
+    mRows.mGridGap =
+      ::ResolveToDefiniteSize(mGridStyle->mGridRowGap, aContentBox.BSize(mWM));
   }
 }
 
 /**
  * (XXX share this utility function with nsFlexContainerFrame at some point)
  *
  * Helper for BuildDisplayList, to implement this special-case for grid
  * items from the spec:
@@ -4953,41 +4941,31 @@ nsGridContainerFrame::Tracks::AlignJusti
 }
 
 nscoord
 nsGridContainerFrame::Tracks::BackComputedIntrinsicSize(
   const TrackSizingFunctions& aFunctions,
   const nsStyleCoord& aGridGap) const
 {
   // Sum up the current sizes (where percentage tracks were treated as 'auto')
-  // in 'size' and a sum of percentages in 'percent'.
+  // in 'size'.
   nscoord size = 0;
-  float percent = 0.0f;
-  bool hasPercent = mStateUnion & TrackSize::eIndefinitePercentMinSizing;
   for (size_t i = 0, len = mSizes.Length(); i < len; ++i) {
-    const nscoord trackSize = mSizes[i].mBase;
-    nscoord length;
-    float p;
-    if (hasPercent &&
-        ::GetPercentSizeParts(aFunctions.MinSizingFor(i), &length, &p)) {
-      size += std::max(length, trackSize);
-      percent += p;
-    } else {
-      size += trackSize;
-    }
-  }
-
-  // Add grid-gap contributions to 'size' and 'percent'.
+    size += mSizes[i].mBase;
+  }
+
+  // Add grid-gap contributions to 'size' and calculate a 'percent' sum.
+  float percent = 0.0f;
   size_t numTracks = mSizes.Length();
   if (numTracks > 1) {
     const size_t gridGapCount = numTracks - 1;
     nscoord gridGapLength;
     float gridGapPercent;
     if (::GetPercentSizeParts(aGridGap, &gridGapLength, &gridGapPercent)) {
-      percent += gridGapCount * gridGapPercent;
+      percent = gridGapCount * gridGapPercent;
     } else {
       gridGapLength = aGridGap.ToLength();
     }
     size += gridGapCount * gridGapLength;
   }
 
   return std::max(0, nsLayoutUtils::AddPercents(size, percent));
 }
@@ -6113,16 +6091,22 @@ nsGridContainerFrame::Reflow(nsPresConte
     InitImplicitNamedAreas(stylePos);
   }
   GridReflowInput gridReflowInput(this, aReflowInput);
   if (gridReflowInput.mIter.ItemsAreAlreadyInOrder()) {
     AddStateBits(NS_STATE_GRID_NORMAL_FLOW_CHILDREN_IN_CSS_ORDER);
   } else {
     RemoveStateBits(NS_STATE_GRID_NORMAL_FLOW_CHILDREN_IN_CSS_ORDER);
   }
+  if (gridReflowInput.mIter.AtEnd()) {
+    // We have no grid items, our parent should synthesize a baseline if needed.
+    AddStateBits(NS_STATE_GRID_SYNTHESIZE_BASELINE);
+  } else {
+    RemoveStateBits(NS_STATE_GRID_SYNTHESIZE_BASELINE);
+  }
   const nscoord computedBSize = aReflowInput.ComputedBSize();
   const nscoord computedISize = aReflowInput.ComputedISize();
   const WritingMode& wm = gridReflowInput.mWM;
   LogicalSize computedSize(wm, computedISize, computedBSize);
 
   nscoord consumedBSize = 0;
   nscoord bSize;
   if (!prevInFlow) {
@@ -6733,21 +6717,18 @@ nsGridContainerFrame::SynthesizeBaseline
   nscoord start;
   nscoord size;
   if (aAxis == eLogicalAxisBlock) {
     start = child->GetLogicalNormalPosition(aCBWM, aCBPhysicalSize).B(aCBWM);
     size = child->BSize(aCBWM);
     if (grid && aGridOrderItem.mIsInEdgeTrack) {
       isOrthogonal ? grid->GetIBaseline(aGroup, &baseline) :
                      grid->GetBBaseline(aGroup, &baseline);
-    } else if (!isOrthogonal && aGridOrderItem.mIsInEdgeTrack &&
-               GetBBaseline(aGroup, childWM, child, &baseline)) {
-      if (aGroup == BaselineSharingGroup::eLast) {
-        baseline = size - baseline; // convert to distance from border-box end
-      }
+    } else if (!isOrthogonal && aGridOrderItem.mIsInEdgeTrack) {
+      baseline = child->BaselineBOffset(childWM, aGroup, AlignmentContext::eGrid);
     } else {
       baseline = ::SynthesizeBaselineFromBorderBox(aGroup, childWM, size);
     }
   } else {
     start = child->GetLogicalNormalPosition(aCBWM, aCBPhysicalSize).I(aCBWM);
     size = child->ISize(aCBWM);
     if (grid && aGridOrderItem.mIsInEdgeTrack) {
       isOrthogonal ? grid->GetBBaseline(aGroup, &baseline) :

--- a/layout/generic/nsGridContainerFrame.h
+++ b/layout/generic/nsGridContainerFrame.h
@@ -10,24 +10,16 @@
 #define nsGridContainerFrame_h___
 
 #include "mozilla/Maybe.h"
 #include "mozilla/TypeTraits.h"
 #include "nsContainerFrame.h"
 #include "nsHashKeys.h"
 #include "nsTHashtable.h"
 
-// https://drafts.csswg.org/css-align-3/#baseline-sharing-group
-enum BaselineSharingGroup
-{
-  // NOTE Used as an array index so must be 0 and 1.
-  eFirst = 0,
-  eLast = 1,
-};
-
 /**
  * Factory function.
  * @return a newly allocated nsGridContainerFrame (infallible)
  */
 nsContainerFrame* NS_NewGridContainerFrame(nsIPresShell* aPresShell,
                                            nsStyleContext* aContext);
 
 namespace mozilla {
@@ -106,21 +98,41 @@ public:
   }
 
   void BuildDisplayList(nsDisplayListBuilder*   aBuilder,
                         const nsRect&           aDirtyRect,
                         const nsDisplayListSet& aLists) override;
 
   nscoord GetLogicalBaseline(mozilla::WritingMode aWM) const override
   {
+    if (HasAnyStateBits(NS_STATE_GRID_SYNTHESIZE_BASELINE)) {
+      // Return a baseline synthesized from our margin-box.
+      return nsContainerFrame::GetLogicalBaseline(aWM);
+    }
     nscoord b;
     GetBBaseline(BaselineSharingGroup::eFirst, &b);
     return b;
   }
 
+  bool GetVerticalAlignBaseline(mozilla::WritingMode aWM,
+                                nscoord* aBaseline) const override
+  {
+    return GetNaturalBaselineBOffset(aWM, BaselineSharingGroup::eFirst, aBaseline);
+  }
+
+  bool GetNaturalBaselineBOffset(mozilla::WritingMode aWM,
+                                 BaselineSharingGroup aBaselineGroup,
+                                 nscoord*             aBaseline) const override
+  {
+    if (HasAnyStateBits(NS_STATE_GRID_SYNTHESIZE_BASELINE)) {
+      return false;
+    }
+    return GetBBaseline(aBaselineGroup, aBaseline);
+  }
+
 #ifdef DEBUG_FRAME_DUMP
   nsresult GetFrameName(nsAString& aResult) const override;
 #endif
 
   // nsContainerFrame overrides
   bool DrainSelfOverflowList() override;
   void AppendFrames(ChildListID aListID, nsFrameList& aFrameList) override;
   void InsertFrames(ChildListID aListID, nsIFrame* aPrevFrame,

--- a/layout/generic/nsIFrame.h
+++ b/layout/generic/nsIFrame.h
@@ -412,16 +412,34 @@ enum nsBidiDirection {
   NSBIDI_LTR,
   /** All right-to-left text This is a 1 value. */
   NSBIDI_RTL,
   /** Mixed-directional text. */
   NSBIDI_MIXED
 };
 
 namespace mozilla {
+
+// https://drafts.csswg.org/css-align-3/#baseline-sharing-group
+enum BaselineSharingGroup
+{
+  // NOTE Used as an array index so must be 0 and 1.
+  eFirst = 0,
+  eLast = 1,
+};
+
+// Loosely: https://drafts.csswg.org/css-align-3/#shared-alignment-context
+enum class AlignmentContext
+{
+  eInline,
+  eTable,
+  eFlexbox,
+  eGrid,
+};
+
 /*
  * For replaced elements only. Gets the intrinsic dimensions of this element.
  * The dimensions may only be one of the following two types:
  *
  *   eStyleUnit_Coord   - a length in app units
  *   eStyleUnit_None    - the element has no intrinsic size in this dimension
  */
 struct IntrinsicSize {
@@ -494,16 +512,18 @@ static void ReleaseValue(T* aPropertyVal
  * link to many of the functions defined here. Too bad.
  *
  * If you're not in layout but you must call functions in here, at least
  * restrict yourself to calling virtual methods, which won't hurt you as badly.
  */
 class nsIFrame : public nsQueryFrame
 {
 public:
+  using AlignmentContext = mozilla::AlignmentContext;
+  using BaselineSharingGroup = mozilla::BaselineSharingGroup;
   template <typename T> using Maybe = mozilla::Maybe<T>;
   using Nothing = mozilla::Nothing;
   using OnNonvisible = mozilla::OnNonvisible;
   template<typename T=void>
   using PropertyDescriptor = const mozilla::FramePropertyDescriptor<T>*;
   using ReflowInput = mozilla::ReflowInput;
   using ReflowOutput = mozilla::ReflowOutput;
   using Visibility = mozilla::Visibility;
@@ -1194,21 +1214,103 @@ public:
                               nscoord aRadii[8]) const;
   bool GetBorderRadii(nscoord aRadii[8]) const;
   bool GetMarginBoxBorderRadii(nscoord aRadii[8]) const;
   bool GetPaddingBoxBorderRadii(nscoord aRadii[8]) const;
   bool GetContentBoxBorderRadii(nscoord aRadii[8]) const;
   bool GetShapeBoxBorderRadii(nscoord aRadii[8]) const;
 
   /**
+   * XXX: this method will likely be replaced by GetVerticalAlignBaseline
    * Get the position of the frame's baseline, relative to the top of
    * the frame (its top border edge).  Only valid when Reflow is not
    * needed.
+   * @note You should only call this on frames with a WM that's parallel to aWM.
+   * @param aWM the writing-mode of the alignment context, with the ltr/rtl
+   * direction tweak done by nsIFrame::GetWritingMode(nsIFrame*) in inline
+   * contexts (see that method).
    */
-  virtual nscoord GetLogicalBaseline(mozilla::WritingMode aWritingMode) const = 0;
+  virtual nscoord GetLogicalBaseline(mozilla::WritingMode aWM) const = 0;
+
+  /**
+   * Synthesize a first(last) inline-axis baseline from our margin-box.
+   * An alphabetical baseline is at the start(end) edge and a central baseline
+   * is at the center of our block-axis margin-box (aWM tells which to use).
+   * https://drafts.csswg.org/css-align-3/#synthesize-baselines
+   * @note You should only call this on frames with a WM that's parallel to aWM.
+   * @param aWM the writing-mode of the alignment context
+   * @return an offset from our border-box block-axis start(end) edge for
+   * a first(last) baseline respectively
+   * (implemented in nsIFrameInlines.h)
+   */
+  inline nscoord SynthesizeBaselineBOffsetFromMarginBox(
+                   mozilla::WritingMode aWM,
+                   BaselineSharingGroup aGroup) const;
+
+  /**
+   * Synthesize a first(last) inline-axis baseline from our border-box.
+   * An alphabetical baseline is at the start(end) edge and a central baseline
+   * is at the center of our block-axis border-box (aWM tells which to use).
+   * https://drafts.csswg.org/css-align-3/#synthesize-baselines
+   * @note The returned value is only valid when reflow is not needed.
+   * @note You should only call this on frames with a WM that's parallel to aWM.
+   * @param aWM the writing-mode of the alignment context
+   * @return an offset from our border-box block-axis start(end) edge for
+   * a first(last) baseline respectively
+   * (implemented in nsIFrameInlines.h)
+   */
+  inline nscoord SynthesizeBaselineBOffsetFromBorderBox(
+                   mozilla::WritingMode aWM,
+                   BaselineSharingGroup aGroup) const;
+
+  /**
+   * Return the position of the frame's inline-axis baseline, or synthesize one
+   * for the given alignment context. The returned baseline is the distance from
+   * the block-axis border-box start(end) edge for aBaselineGroup eFirst(eLast).
+   * @note The returned value is only valid when reflow is not needed.
+   * @note You should only call this on frames with a WM that's parallel to aWM.
+   * @param aWM the writing-mode of the alignment context
+   * @param aBaselineOffset out-param, only valid if the method returns true
+   * (implemented in nsIFrameInlines.h)
+   */
+  inline nscoord BaselineBOffset(mozilla::WritingMode aWM,
+                                 BaselineSharingGroup aBaselineGroup,
+                                 AlignmentContext     aAlignmentContext) const;
+
+  /**
+   * XXX: this method is taking over the role that GetLogicalBaseline has.
+   * Return true if the frame has a CSS2 'vertical-align' baseline.
+   * If it has, then the returned baseline is the distance from the block-
+   * axis border-box start edge.
+   * @note This method should only be used in AlignmentContext::eInline contexts.
+   * @note The returned value is only valid when reflow is not needed.
+   * @note You should only call this on frames with a WM that's parallel to aWM.
+   * @param aWM the writing-mode of the alignment context
+   * @param aBaseline the baseline offset, only valid if the method returns true
+   */
+  virtual bool GetVerticalAlignBaseline(mozilla::WritingMode aWM,
+                                        nscoord* aBaseline) const {
+    return false;
+  }
+
+  /**
+   * Return true if the frame has a first(last) inline-axis natural baseline per
+   * CSS Box Alignment.  If so, then the returned baseline is the distance from
+   * the block-axis border-box start(end) edge for aBaselineGroup eFirst(eLast).
+   * https://drafts.csswg.org/css-align-3/#natural-baseline
+   * @note The returned value is only valid when reflow is not needed.
+   * @note You should only call this on frames with a WM that's parallel to aWM.
+   * @param aWM the writing-mode of the alignment context
+   * @param aBaseline the baseline offset, only valid if the method returns true
+   */
+  virtual bool GetNaturalBaselineBOffset(mozilla::WritingMode aWM,
+                                         BaselineSharingGroup aBaselineGroup,
+                                         nscoord*             aBaseline) const {
+    return false;
+  }
 
   /**
    * Get the position of the baseline on which the caret needs to be placed,
    * relative to the top of the frame.  This is mostly needed for frames
    * which return a baseline from GetBaseline which is not useful for
    * caret positioning.
    */
   virtual nscoord GetCaretBaseline() const {

--- a/layout/generic/nsIFrameInlines.h
+++ b/layout/generic/nsIFrameInlines.h
@@ -91,9 +91,73 @@ nsIFrame::IsInlineOutside() const
 }
 
 mozilla::StyleDisplay
 nsIFrame::GetDisplay() const
 {
   return StyleDisplay()->GetDisplay(this);
 }
 
+nscoord
+nsIFrame::SynthesizeBaselineBOffsetFromMarginBox(
+            mozilla::WritingMode aWM,
+            BaselineSharingGroup aGroup) const
+{
+  MOZ_ASSERT(!aWM.IsOrthogonalTo(GetWritingMode()));
+  auto margin = GetLogicalUsedMargin(aWM);
+  if (aGroup == BaselineSharingGroup::eFirst) {
+    if (aWM.IsAlphabeticalBaseline()) {
+      // First baseline for inverted-line content is the block-start margin edge,
+      // as the frame is in effect "flipped" for alignment purposes.
+      return MOZ_UNLIKELY(aWM.IsLineInverted()) ? -margin.BStart(aWM)
+                                                : BSize(aWM) + margin.BEnd(aWM);
+    }
+    nscoord marginBoxCenter = (BSize(aWM) + margin.BStartEnd(aWM)) / 2;
+    return marginBoxCenter - margin.BStart(aWM);
+  }
+  MOZ_ASSERT(aGroup == BaselineSharingGroup::eLast);
+  if (aWM.IsAlphabeticalBaseline()) {
+    // Last baseline for inverted-line content is the block-start margin edge,
+    // as the frame is in effect "flipped" for alignment purposes.
+    return MOZ_UNLIKELY(aWM.IsLineInverted()) ? BSize(aWM) + margin.BStart(aWM)
+                                              : -margin.BEnd(aWM);
+  }
+  // Round up for central baseline offset, to be consistent with eFirst.
+  nscoord marginBoxSize = BSize(aWM) + margin.BStartEnd(aWM);
+  nscoord marginBoxCenter = (marginBoxSize / 2) + (marginBoxSize % 2);
+  return marginBoxCenter - margin.BEnd(aWM);
+}
+
+nscoord
+nsIFrame::SynthesizeBaselineBOffsetFromBorderBox(
+            mozilla::WritingMode aWM,
+            BaselineSharingGroup aGroup) const
+{
+  MOZ_ASSERT(!aWM.IsOrthogonalTo(GetWritingMode()));
+  nscoord borderBoxSize = BSize(aWM);
+  if (aGroup == BaselineSharingGroup::eFirst) {
+    return MOZ_LIKELY(aWM.IsAlphabeticalBaseline()) ? borderBoxSize
+                                                    : borderBoxSize / 2;
+  }
+  MOZ_ASSERT(aGroup == BaselineSharingGroup::eLast);
+  // Round up for central baseline offset, to be consistent with eFirst.
+  auto borderBoxCenter = (borderBoxSize / 2) + (borderBoxSize % 2);
+  return MOZ_LIKELY(aWM.IsAlphabeticalBaseline()) ? 0 : borderBoxCenter;
+}
+
+nscoord
+nsIFrame::BaselineBOffset(mozilla::WritingMode aWM,
+                          BaselineSharingGroup aBaselineGroup,
+                          AlignmentContext     aAlignmentContext) const
+{
+  MOZ_ASSERT(!aWM.IsOrthogonalTo(GetWritingMode()));
+  nscoord baseline;
+  if (GetNaturalBaselineBOffset(aWM, aBaselineGroup, &baseline)) {
+    return baseline;
+  }
+  if (aAlignmentContext == AlignmentContext::eInline) {
+    return SynthesizeBaselineBOffsetFromMarginBox(aWM, aBaselineGroup);
+  }
+  // XXX AlignmentContext::eTable should use content box?
+  return SynthesizeBaselineBOffsetFromBorderBox(aWM, aBaselineGroup);
+}
+
 #endif

--- a/layout/generic/nsIScrollableFrame.h
+++ b/layout/generic/nsIScrollableFrame.h
@@ -470,11 +470,19 @@ public:
   virtual void TriggerDisplayPortExpiration() = 0;
 
   /**
    * Returns information required to determine where to snap to after a scroll.
    */
   virtual ScrollSnapInfo GetScrollSnapInfo() const = 0;
 
   virtual void SetScrollsClipOnUnscrolledOutOfFlow() = 0;
+
+  /**
+   * Given the drag event aEvent, determine whether the mouse is near the edge
+   * of the scrollable area, and scroll the view in the direction of that edge
+   * if so. If scrolling occurred, true is returned. When false is returned, the
+   * caller should look for an ancestor to scroll.
+   */
+  virtual bool DragScroll(mozilla::WidgetEvent* aEvent) = 0;
 };
 
 #endif

new file mode 100644
--- /dev/null
+++ b/layout/reftests/css-grid/grid-container-synthesized-baseline-001-ref.html
@@ -0,0 +1,46 @@
+<!DOCTYPE HTML>
+<!--
+     Any copyright is dedicated to the Public Domain.
+     http://creativecommons.org/publicdomain/zero/1.0/
+-->
+<html><head>
+  <meta charset="utf-8">
+  <title>Reference: Synthesized grid container baseline.</title>
+  <link rel="author" title="Mats Palmgren" href="https://bugzilla.mozilla.org/show_bug.cgi?id=1313068">
+  <style type="text/css">
+html,body {
+  color:black; background-color:white; font:16px/1 monospace; padding:0; margin:0;
+}
+.ib {
+  display: inline-block;
+}
+.ig {
+  display: inline-grid;
+}
+.ib, .ig {
+  border-style: solid;
+  border-width: 3px 1px 5px 1px;
+  padding: 7px 10px 3px 8px;
+  margin: 5px 3px 2px 1px;
+}
+</style>
+
+</head><body>
+
+<pre>Inline-level context:</pre>
+Grid:<div class="ib"></div>
+Block:<div class="ig"></div>
+
+<pre>Grid-level context:</pre>
+<div style="display:inline-grid; grid-auto-flow:column; align-items:baseline; justify-items:start">
+Grid:<div class="ib"></div>
+Block:<div class="ig"></div>
+</div>
+
+<pre>Flexbox-level context:</pre>
+<div style="display:inline-flex; align-items:baseline; justify-items:start">
+Grid:<div class="ib" style="margin-bottom:0"></div>
+Block:<div class="ig"></div>
+</div>
+
+</body></html>

new file mode 100644
--- /dev/null
+++ b/layout/reftests/css-grid/grid-container-synthesized-baseline-001.html
@@ -0,0 +1,48 @@
+<!DOCTYPE HTML>
+<!--
+     Any copyright is dedicated to the Public Domain.
+     http://creativecommons.org/publicdomain/zero/1.0/
+-->
+<html><head>
+  <meta charset="utf-8">
+  <title>CSS Grid Test: Synthesized grid container baseline.</title>
+  <link rel="author" title="Mats Palmgren" href="https://bugzilla.mozilla.org/show_bug.cgi?id=1313068">
+  <link rel="help" href="https://drafts.csswg.org/css-grid/#grid-baselines">
+  <link rel="match" href="grid-container-synthesized-baseline-001-ref.html">
+  <style type="text/css">
+html,body {
+  color:black; background-color:white; font:16px/1 monospace; padding:0; margin:0;
+}
+.ib {
+  display: inline-block;
+}
+.ig {
+  display: inline-grid;
+}
+.ib, .ig {
+  border-style: solid;
+  border-width: 3px 1px 5px 1px;
+  padding: 7px 10px 3px 8px;
+  margin: 5px 3px 2px 1px;
+}
+</style>
+
+</head><body>
+
+<pre>Inline-level context:</pre>
+Grid:<div class="ig"></div>
+Block:<div class="ib"></div>
+
+<pre>Grid-level context:</pre>
+<div style="display:inline-grid; grid-auto-flow:column; align-items:baseline; justify-items:start">
+Grid:<div class="ig"></div>
+Block:<div class="ib" style="margin-bottom:0"></div>
+</div>
+
+<pre>Flexbox-level context:</pre>
+<div style="display:inline-flex; align-items:baseline; justify-items:start">
+Grid:<div class="ig"></div>
+Block:<div class="ib" style="margin-bottom:0"></div>
+</div>
+
+</body></html>

--- a/layout/reftests/css-grid/grid-fragmentation-015-ref.html
+++ b/layout/reftests/css-grid/grid-fragmentation-015-ref.html
@@ -79,17 +79,17 @@ x { display:block; height:20px; }
 <span style="grid-row:span 2"><x></x></span>
 <span><x></x></span>
 <span><x></x></span>
 </div></div></div></div>
 
 <!-- grid wrapped in FIELDSET inline -->
 <div class="columns" style="height: 40px; margin-left:200px">
 <div style="padding-top:2px; background:grey">
-<div style="display:inline-block; overflow:hidden; border:none; padding:0; margin:0">
+<div style="display:inline-block; border:none; padding:0; margin:0">
 <div class="grid">
 <span style="grid-row:span 2"><x></x></span>
 <span><x></x></span>
 <span><x></x></span>
 </div></div></div></div>
 
 <!-- grid wrapped in FIELDSET overflow:hidden block -->
 <div class="columns" style="height: 40px; margin-left:400px">

--- a/layout/reftests/css-grid/grid-percent-intrinsic-sizing-001-ref.html
+++ b/layout/reftests/css-grid/grid-percent-intrinsic-sizing-001-ref.html
@@ -11,133 +11,65 @@
   <style type="text/css">
 html,body {
     color:black; background-color:white; font:16px/1 monospace; padding:0; margin:0;
 }
 
 div {
   display: grid;
   float: left;
-  grid-template-columns: auto auto;
-  grid-template-rows: 5px;
-  height: 5px;
   border: 1px solid;
   clear: left;
   align-content: start;
   justify-content: start;
+  margin: 3px;
 }
 
 span {
-  min-width: 10px;
   background: grey;
 }
+.c > span { width: 10px; }
+.r > span { height: 10px; }
+
 span:nth-child(2) { background:lime; }
 x { background: blue; }
 x:nth-child(2) { background:pink; }
 
-.g10  { grid-gap:10%; }
-.p1   { grid-template-columns: 10px; width: calc(10px / 0.9); }
-.p1a  { grid-template-columns: 10px; width: calc(10px / 0.9); }
-.p10  { grid-template-columns: 10px 10px; width: calc(10px / 0.8); }
-.p10a { grid-template-columns: calc((20px / 0.5) * 0.4) calc((20px / 0.5) * 0.1); width: calc(20px / 0.5); }
-.p10b { grid-template-columns: calc((110px / 0.6) * 0.4) 100px; width: calc(110px / 0.6); }
-.g10.p10b { grid-template-columns: calc((110px / 0.5) * 0.4) 100px; width: calc(110px / 0.5);  }
-.p10c { grid-template-columns: calc((110px / 0.6) * 0.2) 100px calc((110px / 0.6) * 0.2); width: calc(110px / 0.6); }
-.g10.p10c { grid-template-columns: calc((110px / 0.4) * 0.2) 100px calc((110px / 0.4) * 0.2); width: calc(110px / 0.4); }
-.c10  { grid-template-columns: 62.5px 62.5px; }
-.g10.c10  { grid-template-columns: 64.2833px 64.2833px; }
-.c10120  { grid-template-columns: calc(((170px / 0.8) * 0.1) + 50px) calc(((170px / 0.8) * 0.1) + 50px); width: calc(170px / 0.8); }
-.g10.c10120 { grid-template-columns: calc(((170px / 0.7) * 0.1) + 50px) calc(((170px / 0.7) * 0.1) + 50px); width: calc(170px / 0.7); }
-.c10a { grid-template-columns: calc(((170px / 0.8) * 0.1) + 50px) calc(((170px / 0.8) * 0.1) + 50px); width: calc(170px / 0.8); }
-.g10.c10a { grid-template-columns: calc(((170px / 0.7) * 0.1) + 50px) calc(((170px / 0.7) * 0.1) + 50px); width: calc(170px / 0.7); }
-.c10b { grid-template-columns: calc(50px - (170px * 0.1)) calc(50px - (170px * 0.1)); width: 170px; }
+.c { grid-auto-rows: 5px; }
+.c.p1   { grid-template-columns: 1px; width: 10px; }
+.c.p1a  { grid-template-columns: minmax(10%,auto); }
+.c.p2   { grid-template-columns: 10% 10%; grid-gap: 20%; }
+.c.p2a  { grid-template-columns: repeat(2,minmax(10%,auto)); grid-gap: 20%; }
+.c.c0  { grid-template-columns: 0; }
 
-.p1x   { grid-template-columns: 0; }
-.p1ax  { grid-template-columns: 0; }
-.p10x  { grid-template-columns: auto auto; }
-.p10ax { grid-template-columns: calc((10px / 0.5) * 0.4) calc((10px / 0.5) * 0.1); width: calc(10px / 0.5); }
-.g10.p10 { width: calc(10px / 0.7); }
-.g10.p10a  { grid-template-columns: calc((20px / 0.4) * 0.4) calc((20px / 0.4) * 0.1); width: calc(20px / 0.4); }
-.g10.p10ax { grid-template-columns: calc((10px / 0.4) * 0.4) calc((10px / 0.4) * 0.1); width: calc(10px / 0.4);  }
-.p10axx{ grid-template-columns: auto auto; }
-.p10bx { grid-template-columns: calc((110px / 0.6) * 0.4) 100px; width: calc(110px / 0.6); }
-.p10bx120 { grid-template-columns: calc((100px / 0.6) * 0.4) 100px; width: calc(100px / 0.6); }
-.g10.p10bx { grid-template-columns: calc((110px / 0.5) * 0.4) 100px; width: calc(110px / 0.5); }
-.g10.p10bx1 { grid-template-columns: calc((100px / 0.5) * 0.4) 100px; width: calc(100px / 0.5); }
-.p10bxx{ grid-template-columns: 66.66667px 100px; }
-.g10.p10bxx{ grid-template-columns: 80px 100px; }
-.p10cx { grid-template-columns: calc((110px / 0.6) * 0.2) 100px calc((110px / 0.6) * 0.2); width: calc(110px / 0.6); }
-.g10.p10cx { grid-template-columns: calc((110px / 0.4) * 0.2) 100px calc((110px / 0.4) * 0.2); width: calc(110px / 0.4); }
-.c10x  { grid-template-columns: 62.5px 62.5px; }
-.g10.c10xx  { grid-template-columns: 64.2833px 64.2833px; }
-.g10.c10x  { grid-template-columns: 64.2833px 64.2833px; }
-.c10xx { grid-template-columns: 62.5px 62.5px; }
-.c10ax { grid-template-columns: calc(((170px / 0.8) * 0.1) + 50px) calc(((170px / 0.8) * 0.1) + 50px) ; width: calc(170px / 0.8); }
-.g10.c10ax { grid-template-columns: calc(((170px / 0.7) * 0.1) + 50px) calc(((170px / 0.7) * 0.1) + 50px) ; width: calc(170px / 0.7); }
-.c10bx { grid-template-columns: calc(50px - (170px * 0.1)) calc(50px - (170px * 0.1)); width: 170px; }
+.r { grid-auto-columns: 5px; grid-auto-flow: column; }
+.r.p1   { grid-template-rows: 10%; }
+.r.p1a  { grid-template-rows: minmax(10%,auto); }
+.r.p2   { grid-template-rows: 10% 10%; grid-gap: 20%; }
+.r.p2a  { grid-template-rows: repeat(2,minmax(10%,auto)); grid-gap: 20%; }
+.r.r0   { grid-template-rows: 0; }
 
-.gneg { grid-gap: 0; grid-template-columns: 10px 10px; width:0; }
-.gneg.c10a { grid-template-columns: 50px 50px; }
-.gneg.p10b { grid-template-columns: 8px 100px; width: 20px; }
   </style>
 </head>
 <body>
 
-<!-- Note that some of the min-width cases below SHOULD overflow. -->
+<div class="c p1"><span></span><x></x></div>
+<div class="c c0"><x></x></div>
+<div class="c p1a"><span></span><x></x></div>
+<div class="c c0"><x></x></div>
 
-<div class="p1"><span></span></div>
-<div class="p1x"><x></x></div>
-<div class="p10"><span></span></div>
-<div class="p10x"><x></x></div>
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-<div class="p10bx120"><x></x><span style="min-width:120px"></span></div>
-<div class="p10c"><span></span><span></span></div>
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-<div class="c10b"><span></span><span style="min-width:120px"></span></div>
-<div class="c10bx"><x></x><span style="min-width:120px"></span></div>
-
+<div class="c p2"><span></span><span></span><x></x></div>
+<div class="c c0"><x></x></div>
+<div class="c p2a"><span></span><span></span><x></x></div>
+<div class="c c0"><x></x></div>
 
-<div class="g10"><span></span><span></span></div>
-<div class="g10 p1"><span></span></div>
-<div class="g10 p1x"><x></x></div>
-<div class="g10 p10"><span></span></div>
-<div class="g10 p10x"><x></x></div>
-<div class="g10 p1a"><span></span></div>
-<div class="g10 p1ax"><x></x></div>
-<div class="g10 p10a"><span></span><span></span></div>
-<div class="g10 p10ax"><x></x><span></span></div>
-<div class="g10 p10axx"><x></x><x></x></div>
-<div class="g10 p10bx"><span></span><span></span></div>
-<div class="g10 p10bx"><span></span><x></x></div>
-<div class="g10 p10bxx"><x></x><x></x></div>
-<div class="g10 p10b"><span></span><span style="min-width:80px"></span></div>
-<div class="g10 p10bx1"><x></x><span style="min-width:120px"></span></div>
-<div class="g10 p10c"><span></span><span></span></div>
-<div class="g10 p10cx"><span></span><x></x></div>
-<div class="g10 c10"><span></span><span></span></div>
-<div class="g10 c10xx"><x></x><x></x></div>
-<div class="g10 c10120"><span></span><span style="min-width:120px"></span></div>
-<div class="g10 c10x"><x></x><span></span></div>
-<div class="g10 c10a"><span></span><span style="min-width:120px"></span></div>
-<div class="g10 c10ax"><x></x><span style="min-width:120px"></span></div>
-<div class="g10 c10b"><span></span><span style="min-width:120px"></span></div>
-<div class="g10 c10bx"><x></x><span style="min-width:120px"></span></div>
+<div class="r p1"><span></span><x></x></div>
+<div class="r r0"><x></x></div>
+<div class="r p1a"><span></span><x></x></div>
+<div class="r r0"><x></x></div>
 
-<div class="gneg"><span></span><span></span></div>
-<div class="gneg c10a"><span></span><span></span></div>
-<div class="gneg p10b"><span></span><span></span></div>
+<div class="r p2"><span></span><span></span><x></x></div>
+<div class="r r0"><x></x></div>
+<div class="r p2a"><span></span><span></span><x></x></div>
+<div class="r r0"><x></x></div>
 
 </body>
 </html>