--- a/layout/base/nsLayoutUtils.cpp
+++ b/layout/base/nsLayoutUtils.cpp
@@ -755,16 +755,23 @@ nsLayoutUtils::GetDisplayPort(nsIContent
         *aResult = result;
       }
     }
   }
 
   return true;
 }
 
+void
+nsLayoutUtils::SetDisplayPortBase(nsIContent* aContent, const nsRect& aBase)
+{
+  aContent->SetProperty(nsGkAtoms::DisplayPortBase, new nsRect(aBase),
+                        nsINode::DeleteProperty<nsRect>);
+}
+
 bool
 nsLayoutUtils::GetCriticalDisplayPort(nsIContent* aContent, nsRect* aResult)
 {
   void* property = aContent->GetProperty(nsGkAtoms::CriticalDisplayPort);
   if (!property) {
     return false;
   }
 
@@ -2368,17 +2375,22 @@ nsLayoutUtils::PaintFrame(nsRenderingCon
   }
 
   nsIFrame* rootScrollFrame = presShell->GetRootScrollFrame();
   bool usingDisplayPort = false;
   nsRect displayport;
   if (rootScrollFrame && !aFrame->GetParent()) {
     nsIContent* content = rootScrollFrame->GetContent();
     if (content) {
-      usingDisplayPort = nsLayoutUtils::GetDisplayPort(content, &displayport);
+      usingDisplayPort = nsLayoutUtils::GetDisplayPort(content);
+      if (usingDisplayPort) {
+        nsLayoutUtils::SetDisplayPortBase(content,
+          nsRect(nsPoint(0,0), nsLayoutUtils::CalculateCompositionSizeForFrame(rootScrollFrame)));
+        nsLayoutUtils::GetDisplayPort(content, &displayport);
+      }
     }
   }
 
   nsRegion visibleRegion;
   if (aFlags & PAINT_WIDGET_LAYERS) {
     // This layer tree will be reused, so we'll need to calculate it
     // for the whole "visible" area of the window
     // 

--- a/layout/base/nsLayoutUtils.h
+++ b/layout/base/nsLayoutUtils.h
@@ -145,16 +145,22 @@ public:
    */
   static nsIScrollableFrame* FindScrollableFrameFor(ViewID aId);
 
   /**
    * Get display port for the given element.
    */
   static bool GetDisplayPort(nsIContent* aContent, nsRect *aResult = nullptr);
 
+ /**
+  * Set the display port base rect for given element to be used with display
+  * port margins.
+  */
+ static void SetDisplayPortBase(nsIContent* aContent, const nsRect& aBase);
+
   /**
    * Get the critical display port for the given element.
    */
   static bool GetCriticalDisplayPort(nsIContent* aContent, nsRect* aResult = nullptr);
 
   /**
    * Use heuristics to figure out the child list that
    * aChildFrame is currently in.

--- a/layout/generic/nsGfxScrollFrame.cpp
+++ b/layout/generic/nsGfxScrollFrame.cpp
@@ -2453,20 +2453,31 @@ ScrollFrameHelper::BuildDisplayList(nsDi
   // Not all our descendants will be clipped by overflow clipping, but all
   // the ones that aren't clipped will be out of flow frames that have already
   // had dirty rects saved for them by their parent frames calling
   // MarkOutOfFlowChildrenForDisplayList, so it's safe to restrict our
   // dirty rect here.
   nsRect dirtyRect = aDirtyRect.Intersect(mScrollPort);
 
   // Override the dirty rectangle if the displayport has been set.
+  bool usingDisplayport =
+    nsLayoutUtils::GetDisplayPort(mOuter->GetContent()) &&
+    !aBuilder->IsForEventDelivery();
+
+  // don't set the display port base rect for root scroll frames,
+  // nsLayoutUtils::PaintFrame or nsSubDocumentFrame::BuildDisplayList
+  // does that for root scroll frames before it expands the dirty rect
+  // to the display port.
+  if (usingDisplayport && !mIsRoot) {
+    nsLayoutUtils::SetDisplayPortBase(mOuter->GetContent(), dirtyRect);
+  }
+
+  // now that we have an updated base rect we can get the display port
   nsRect displayPort;
-  bool usingDisplayport =
-    nsLayoutUtils::GetDisplayPort(mOuter->GetContent(), &displayPort) &&
-    !aBuilder->IsForEventDelivery();
+  nsLayoutUtils::GetDisplayPort(mOuter->GetContent(), &displayPort);
   if (usingDisplayport && DisplayportExceedsMaxTextureSize(mOuter->PresContext(), displayPort)) {
     usingDisplayport = false;
   }
   if (usingDisplayport) {
     dirtyRect = displayPort;
   }
 
   if (aBuilder->IsForImageVisibility()) {

--- a/layout/generic/nsSubDocumentFrame.cpp
+++ b/layout/generic/nsSubDocumentFrame.cpp
@@ -371,28 +371,34 @@ nsSubDocumentFrame::BuildDisplayList(nsD
   int32_t parentAPD = PresContext()->AppUnitsPerDevPixel();
   int32_t subdocAPD = presContext->AppUnitsPerDevPixel();
 
   nsRect dirty;
   bool haveDisplayPort = false;
   bool ignoreViewportScrolling = false;
   nsIFrame* savedIgnoreScrollFrame = nullptr;
   if (subdocRootFrame) {
-    nsRect displayPort;
-    if (nsLayoutUtils::ViewportHasDisplayPort(presContext, &displayPort)) {
+    // get the dirty rect relative to the root frame of the subdoc
+    dirty = aDirtyRect + GetOffsetToCrossDoc(subdocRootFrame);
+    // and convert into the appunits of the subdoc
+    dirty = dirty.ConvertAppUnitsRoundOut(parentAPD, subdocAPD);
+
+    nsIFrame* rootScrollFrame = presShell->GetRootScrollFrame();
+    if (nsLayoutUtils::ViewportHasDisplayPort(presContext)) {
       haveDisplayPort = true;
+      // for root content documents we want the base to be the composition bounds
+      nsLayoutUtils::SetDisplayPortBase(rootScrollFrame->GetContent(),
+        presContext->IsRootContentDocument() ?
+          nsRect(nsPoint(0,0), nsLayoutUtils::CalculateCompositionSizeForFrame(rootScrollFrame)) :
+          dirty);
+      nsRect displayPort;
+      nsLayoutUtils::ViewportHasDisplayPort(presContext, &displayPort);
       dirty = displayPort;
-    } else {
-      // get the dirty rect relative to the root frame of the subdoc
-      dirty = aDirtyRect + GetOffsetToCrossDoc(subdocRootFrame);
-      // and convert into the appunits of the subdoc
-      dirty = dirty.ConvertAppUnitsRoundOut(parentAPD, subdocAPD);
     }
 
-    nsIFrame* rootScrollFrame = presShell->GetRootScrollFrame();
     ignoreViewportScrolling =
       rootScrollFrame && presShell->IgnoringViewportScrolling();
     if (ignoreViewportScrolling) {
       savedIgnoreScrollFrame = aBuilder->GetIgnoreScrollFrame();
       aBuilder->SetIgnoreScrollFrame(rootScrollFrame);
     }
 
     aBuilder->EnterPresShell(subdocRootFrame, dirty);