--- a/layout/base/PresShell.cpp
+++ b/layout/base/PresShell.cpp
@@ -1906,65 +1906,83 @@ PresShell::sPaintSuppressionCallback(nsI
 
 void
 PresShell::AsyncResizeEventCallback(nsITimer* aTimer, void* aPresShell)
 {
   static_cast<PresShell*>(aPresShell)->FireResizeEvent();
 }
 
 nsresult
-PresShell::ResizeReflow(nscoord aWidth, nscoord aHeight, nscoord aOldWidth, nscoord aOldHeight)
+PresShell::ResizeReflow(nscoord aWidth, nscoord aHeight, nscoord aOldWidth,
+                        nscoord aOldHeight, ResizeReflowOptions aOptions)
 {
   if (mZoomConstraintsClient) {
     // If we have a ZoomConstraintsClient and the available screen area
     // changed, then we might need to disable double-tap-to-zoom, so notify
     // the ZCC to update itself.
     mZoomConstraintsClient->ScreenSizeChanged();
   }
   if (mMobileViewportManager) {
     // If we have a mobile viewport manager, request a reflow from it. It can
     // recompute the final CSS viewport and trigger a call to
     // ResizeReflowIgnoreOverride if it changed.
     mMobileViewportManager->RequestReflow();
     return NS_OK;
   }
 
-  return ResizeReflowIgnoreOverride(aWidth, aHeight, aOldWidth, aOldHeight);
+  return ResizeReflowIgnoreOverride(aWidth, aHeight, aOldWidth,
+                                    aOldHeight, aOptions);
 }
 
 nsresult
-PresShell::ResizeReflowIgnoreOverride(nscoord aWidth, nscoord aHeight, nscoord aOldWidth, nscoord aOldHeight)
+PresShell::ResizeReflowIgnoreOverride(nscoord aWidth, nscoord aHeight,
+                                      nscoord aOldWidth, nscoord aOldHeight,
+                                      ResizeReflowOptions aOptions)
 {
   NS_PRECONDITION(!mIsReflowing, "Shouldn't be in reflow here!");
 
-  // If we don't have a root frame yet, that means we haven't had our initial
-  // reflow... If that's the case, and aWidth or aHeight is unconstrained,
-  // ignore them altogether.
   nsIFrame* rootFrame = mFrameConstructor->GetRootFrame();
-  if (!rootFrame &&
-      (aHeight == NS_UNCONSTRAINEDSIZE || aWidth == NS_UNCONSTRAINEDSIZE)) {
-    // We can't do the work needed for SizeToContent without a root
-    // frame, and we want to return before setting the visible area.
-    return NS_ERROR_NOT_AVAILABLE;
-  }
-
-  mPresContext->SetVisibleArea(nsRect(0, 0, aWidth, aHeight));
-
-  // There isn't anything useful we can do if the initial reflow hasn't happened.
   if (!rootFrame) {
+    // If we don't have a root frame yet, that means we haven't had our initial
+    // reflow... If that's the case, and aWidth or aHeight is unconstrained,
+    // ignore them altogether.
+    if (aHeight == NS_UNCONSTRAINEDSIZE || aWidth == NS_UNCONSTRAINEDSIZE) {
+      // We can't do the work needed for SizeToContent without a root
+      // frame, and we want to return before setting the visible area.
+      return NS_ERROR_NOT_AVAILABLE;
+    }
+
+    mPresContext->SetVisibleArea(nsRect(0, 0, aWidth, aHeight));
+    // There isn't anything useful we can do if the initial reflow hasn't
+    // happened.
     return NS_OK;
   }
 
   WritingMode wm = rootFrame->GetWritingMode();
+  const bool shrinkToFit = aOptions == ResizeReflowOptions::eBSizeLimit;
   NS_PRECONDITION((wm.IsVertical() ? aHeight : aWidth) != NS_UNCONSTRAINEDSIZE,
                   "shouldn't use unconstrained isize anymore");
-
-  const bool isBSizeChanging = wm.IsVertical()
-                               ? aOldWidth != aWidth
-                               : aOldHeight != aHeight;
+  NS_PRECONDITION((wm.IsVertical() ? aWidth : aHeight) !=
+                  NS_UNCONSTRAINEDSIZE || shrinkToFit,
+                  "shouldn't use unconstrained bsize to SizeToContent anymore");
+  bool isBSizeChanging = wm.IsVertical() ? aOldWidth != aWidth
+                                         : aOldHeight != aHeight;
+  nscoord targetWidth = aWidth;
+  nscoord targetHeight = aHeight;
+
+  if (shrinkToFit) {
+    if (wm.IsVertical()) {
+      targetWidth = NS_UNCONSTRAINEDSIZE;
+    } else {
+      targetHeight = NS_UNCONSTRAINEDSIZE;
+    }
+    isBSizeChanging = true;
+  }
+
+  mPresContext->SetVisibleArea(nsRect(0, 0, targetWidth, targetHeight));
 
   RefPtr<nsViewManager> viewManager = mViewManager;
   nsCOMPtr<nsIPresShell> kungFuDeathGrip(this);
 
   if (!GetPresContext()->SuppressingResizeReflow()) {
     // Have to make sure that the content notifications are flushed before we
     // start messing with the frame model; otherwise we can get content doubling.
     mDocument->FlushPendingNotifications(FlushType::ContentAndNotify);
@@ -1993,32 +2011,56 @@ PresShell::ResizeReflowIgnoreOverride(ns
         WillDoReflow();
 
         // Kick off a top-down reflow
         AUTO_LAYOUT_PHASE_ENTRY_POINT(GetPresContext(), Reflow);
         nsViewManager::AutoDisableRefresh refreshBlocker(viewManager);
 
         mDirtyRoots.RemoveElement(rootFrame);
         DoReflow(rootFrame, true);
-      }
+
+        if (shrinkToFit) {
+          const bool reflowAgain = wm.IsVertical() ?
+                                mPresContext->GetVisibleArea().width > aWidth :
+                                mPresContext->GetVisibleArea().height > aHeight;
+
+          if (reflowAgain) {
+            mPresContext->SetVisibleArea(nsRect(0, 0, aWidth, aHeight));
+            DoReflow(rootFrame, true);
+          }
+        }
+      }
+
+      // the first DoReflow above should've set our bsize if it was
+      // NS_UNCONSTRAINEDSIZE, and the isize shouldn't be NS_UNCONSTRAINEDSIZE
+      // anyway
+      NS_ASSERTION(
+        mPresContext->GetVisibleArea().width != NS_UNCONSTRAINEDSIZE,
+        "width should not be NS_UNCONSTRAINEDSIZE after reflow");
+      NS_ASSERTION(
+        mPresContext->GetVisibleArea().height != NS_UNCONSTRAINEDSIZE,
+        "height should not be NS_UNCONSTRAINEDSIZE after reflow");
 
       DidDoReflow(true);
     }
   }
 
   rootFrame = mFrameConstructor->GetRootFrame();
   if (rootFrame) {
     wm = rootFrame->GetWritingMode();
+    // reflow did not happen; if the reflow happened, our bsize should not be
+    // NS_UNCONSTRAINEDSIZE because DoReflow will fix it up to the same values
+    // as below
     if (wm.IsVertical()) {
-      if (aWidth == NS_UNCONSTRAINEDSIZE) {
+      if (mPresContext->GetVisibleArea().width == NS_UNCONSTRAINEDSIZE) {
         mPresContext->SetVisibleArea(
           nsRect(0, 0, rootFrame->GetRect().width, aHeight));
       }
     } else {
-      if (aHeight == NS_UNCONSTRAINEDSIZE) {
+      if (mPresContext->GetVisibleArea().height == NS_UNCONSTRAINEDSIZE) {
         mPresContext->SetVisibleArea(
           nsRect(0, 0, aWidth, rootFrame->GetRect().height));
       }
     }
   }
 
   if (!mIsDestroying && !mResizeEvent.IsPending() &&
       !mAsyncResizeTimerIsActive) {

--- a/layout/base/PresShell.h
+++ b/layout/base/PresShell.h
@@ -106,18 +106,24 @@ public:
   NS_IMETHOD ScrollSelectionIntoView(RawSelectionType aRawSelectionType,
                                      SelectionRegion aRegion,
                                      int16_t aFlags) override;
   NS_IMETHOD RepaintSelection(RawSelectionType aRawSelectionType) override;
 
   virtual void BeginObservingDocument() override;
   virtual void EndObservingDocument() override;
   virtual nsresult Initialize(nscoord aWidth, nscoord aHeight) override;
-  virtual nsresult ResizeReflow(nscoord aWidth, nscoord aHeight, nscoord aOldWidth = 0, nscoord aOldHeight = 0) override;
-  virtual nsresult ResizeReflowIgnoreOverride(nscoord aWidth, nscoord aHeight, nscoord aOldWidth, nscoord aOldHeight) override;
+  virtual nsresult ResizeReflow(nscoord aWidth, nscoord aHeight,
+                                nscoord aOldWidth = 0, nscoord aOldHeight = 0,
+                                ResizeReflowOptions aOptions =
+                                ResizeReflowOptions::eBSizeExact) override;
+  virtual nsresult ResizeReflowIgnoreOverride(nscoord aWidth, nscoord aHeight,
+                                              nscoord aOldWidth, nscoord aOldHeight,
+                                              ResizeReflowOptions aOptions =
+                                              ResizeReflowOptions::eBSizeExact) override;
   virtual nsIPageSequenceFrame* GetPageSequenceFrame() const override;
   virtual nsCanvasFrame* GetCanvasFrame() const override;
 
   virtual void FrameNeedsReflow(nsIFrame *aFrame, IntrinsicDirty aIntrinsicDirty,
                                 nsFrameState aBitToAdd,
                                 ReflowRootHandling aRootHandling =
                                   eInferFromBitToAdd) override;
   virtual void FrameNeedsToContinueReflow(nsIFrame *aFrame) override;

--- a/layout/base/nsDocumentViewer.cpp
+++ b/layout/base/nsDocumentViewer.cpp
@@ -3552,47 +3552,26 @@ nsDocumentViewer::GetContentSizeInternal
   {
     RefPtr<gfxContext> rcx(presShell->CreateReferenceRenderingContext());
     prefWidth = root->GetPrefISize(rcx);
   }
   if (prefWidth > aMaxWidth) {
     prefWidth = aMaxWidth;
   }
 
-  nsAutoPtr<nsPresState> frameState;
-  nsIScrollableFrame *scrollFrame = presShell->GetRootScrollFrameAsScrollable();
-  nsIStatefulFrame *statefulFrame = do_QueryFrame(scrollFrame);
-  if (statefulFrame) {
-    statefulFrame->SaveState(getter_Transfers(frameState));
-  }
-
-  nsresult rv = presShell->ResizeReflow(prefWidth, NS_UNCONSTRAINEDSIZE);
+  nsresult rv = presShell->ResizeReflow(prefWidth, aMaxHeight, 0, 0,
+                                        nsIPresShell::ResizeReflowOptions::eBSizeLimit);
   NS_ENSURE_SUCCESS(rv, rv);
 
   RefPtr<nsPresContext> presContext;
   GetPresContext(getter_AddRefs(presContext));
   NS_ENSURE_TRUE(presContext, NS_ERROR_FAILURE);
 
-  // so how big is it?
+  // Protect against bogus returns here
   nsRect shellArea = presContext->GetVisibleArea();
-  if (shellArea.height > aMaxHeight) {
-    // Reflow to max height if we would up too tall.
-    rv = presShell->ResizeReflow(prefWidth, aMaxHeight);
-    NS_ENSURE_SUCCESS(rv, rv);
-
-    shellArea = presContext->GetVisibleArea();
-
-    // the first reflow reset our scroll, now set it back
-    if (frameState && presShell->GetRootScrollFrameAsScrollable() == scrollFrame) {
-      statefulFrame->RestoreState(frameState);
-      scrollFrame->ScrollToRestoredPosition();
-    }
-  }
-
-  // Protect against bogus returns here
   NS_ENSURE_TRUE(shellArea.width != NS_UNCONSTRAINEDSIZE &&
                  shellArea.height != NS_UNCONSTRAINEDSIZE,
                  NS_ERROR_FAILURE);
 
   *aWidth = presContext->AppUnitsToDevPixels(shellArea.width);
   *aHeight = presContext->AppUnitsToDevPixels(shellArea.height);
 
   return NS_OK;

--- a/layout/base/nsIPresShell.h
+++ b/layout/base/nsIPresShell.h
@@ -357,26 +357,40 @@ public:
    * The coordinates for aWidth and aHeight must be in standard nscoords.
    *
    * Callers of this method must hold a reference to this shell that
    * is guaranteed to survive through arbitrary script execution.
    * Calling Initialize can execute arbitrary script.
    */
   virtual nsresult Initialize(nscoord aWidth, nscoord aHeight) = 0;
 
+  enum class ResizeReflowOptions : uint32_t {
+    // the resulting BSize should be exactly as given
+    eBSizeExact,
+    // the resulting BSize can be less than the given one, producing
+    // shrink-to-fit sizing in the block dimension
+    eBSizeLimit
+  };
   /**
    * Reflow the frame model into a new width and height.  The
    * coordinates for aWidth and aHeight must be in standard nscoord's.
    */
-  virtual nsresult ResizeReflow(nscoord aWidth, nscoord aHeight, nscoord aOldWidth = 0, nscoord aOldHeight = 0) = 0;
+  virtual nsresult ResizeReflow(nscoord aWidth, nscoord aHeight,
+                                nscoord aOldWidth = 0, nscoord aOldHeight = 0,
+                                ResizeReflowOptions aOptions =
+                                  ResizeReflowOptions::eBSizeExact) = 0;
   /**
    * Do the same thing as ResizeReflow but even if ResizeReflowOverride was
    * called previously.
    */
-  virtual nsresult ResizeReflowIgnoreOverride(nscoord aWidth, nscoord aHeight, nscoord aOldWidth, nscoord aOldHeight) = 0;
+  virtual nsresult ResizeReflowIgnoreOverride(
+                     nscoord aWidth, nscoord aHeight,
+                     nscoord aOldWidth, nscoord aOldHeight,
+                     ResizeReflowOptions aOptions =
+                       ResizeReflowOptions::eBSizeExact) = 0;
 
   /**
    * Returns true if ResizeReflowOverride has been called.
    */
   virtual bool GetIsViewportOverridden() = 0;
 
   /**
    * Return true if the presshell expects layout flush.