Bug 1080205. Part 1: Don't add clip rects to async-scrolled layers for root scrollframes, since the FrameMetrics to scroll those layers will be set on a ContainerLayer parent. r=tn

--HG--
extra : rebase_source : b8fc0e3c97d5be50443734ed80caafabd37cf1a3

layout/generic/nsGfxScrollFrame.cpp

@@ -2800,6 +2800,7 @@ ScrollFrameHelper::BuildDisplayList(nsDisplayListBuilder*   aBuilder,
     mOuter->PresContext()->IsRootContentDocument();
 
   if (aBuilder->GetIgnoreScrollFrame() == mOuter || IsIgnoringViewportClipping()) {
+    mAddClipRectToLayer = false;
 
     // If we are a root scroll frame that has a display port we want to add
     // scrollbars, they will be children of the scrollable layer, but they get
@@ -2829,6 +2830,9 @@ ScrollFrameHelper::BuildDisplayList(nsDisplayListBuilder*   aBuilder,
     return;
   }
 
+  mAddClipRectToLayer =
+    !(mIsRoot && mOuter->PresContext()->PresShell()->GetIsViewportOverridden());
+
   // Overflow clipping can never clip frames outside our subtree, so there
   // is no need to worry about whether we are a moving frame that might clip
   // non-moving frames.
@@ -3049,7 +3053,9 @@ ScrollFrameHelper::ComputeFrameMetrics(Layer* aLayer,
 {
   nsRect scrollport = mScrollPort +
     mOuter->GetOffsetToCrossDoc(aContainerReferenceFrame);
-  if (!(mIsRoot && mOuter->PresContext()->PresShell()->GetIsViewportOverridden())) {
+  // Root scrollframes have FrameMetrics and clipping on their container layers,
+  // so don't also apply clipping here.
+  if (mAddClipRectToLayer) {
     *aClipRect = scrollport;
   }
 

layout/generic/nsGfxScrollFrame.h

@@ -450,6 +450,8 @@ public:
   // If true, the layer should always be active because we always build a
   // scrollable layer. Used for asynchronous scrolling.
   bool mShouldBuildScrollableLayer:1;
+  // If true, add clipping in ScrollFrameHelper::ComputeFrameMetrics.
+  bool mAddClipRectToLayer:1;
 
   // True if this frame has been scrolled at least once
   bool mHasBeenScrolled:1;