Bug 1298218 - Save and restore the combined clip for the "top layer". r=tnikkel

MozReview-Commit-ID: IRfB85gVyWB

--HG--
extra : rebase_source : 15cfdd7f2f09e3a3d09887ef9858f1e28b90c94f

layout/generic/ViewportFrame.cpp

@@ -107,8 +107,15 @@ BuildDisplayListForTopLayerFrame(nsDisplayListBuilder* aBuilder,
     savedOutOfFlowData = nsDisplayListBuilder::GetOutOfFlowData(aFrame);
   if (savedOutOfFlowData) {
     dirty = savedOutOfFlowData->mDirtyRect;
+    // This function is called after we've finished building display items for
+    // the root scroll frame. That means that the content clip from the root
+    // scroll frame is no longer on aBuilder. However, we need to make sure
+    // that the display items we build in this function have finite clipped
+    // bounds with respect to the root ASR, so we restore the *combined clip*
+    // that we saved earlier. The combined clip will include the clip from the
+    // root scroll frame.
     clipState.SetClipChainForContainingBlockDescendants(
-      savedOutOfFlowData->mContainingBlockClipChain);
+      savedOutOfFlowData->mCombinedClipChain);
     clipState.ClipContainingBlockDescendantsExtra(
       dirty + aBuilder->ToReferenceFrame(aFrame), nullptr);
     asrSetter.SetCurrentActiveScrolledRoot(

layout/painting/nsDisplayList.cpp

@@ -1051,8 +1051,9 @@ void nsDisplayListBuilder::MarkOutOfFlowFrameForDisplay(nsIFrame* aDirtyFrame,
   // to objects on the stack, so we need to clone the chain.
   const DisplayItemClipChain* clipChain =
     CopyWholeChain(mClipState.GetClipChainForContainingBlockDescendants());
+  const DisplayItemClipChain* combinedClipChain = mClipState.GetCurrentCombinedClipChain(this);
   const ActiveScrolledRoot* asr = mCurrentActiveScrolledRoot;
-  OutOfFlowDisplayData* data = new OutOfFlowDisplayData(clipChain, asr, dirty);
+  OutOfFlowDisplayData* data = new OutOfFlowDisplayData(clipChain, combinedClipChain, asr, dirty);
   aFrame->Properties().Set(nsDisplayListBuilder::OutOfFlowDisplayDataProperty(), data);
 
   MarkFrameForDisplay(aFrame, aDirtyFrame);
@@ -1275,9 +1276,10 @@ nsDisplayListBuilder::MarkFramesForDisplayList(nsIFrame* aDirtyFrame,
 
     const DisplayItemClipChain* clipChain =
       CopyWholeChain(mClipState.GetClipChainForContainingBlockDescendants());
+    const DisplayItemClipChain* combinedClipChain = mClipState.GetCurrentCombinedClipChain(this);
     const ActiveScrolledRoot* asr = mCurrentActiveScrolledRoot;
     CurrentPresShellState()->mFixedBackgroundDisplayData.emplace(
-      clipChain, asr, aDirtyRect);
+      clipChain, combinedClipChain, asr, aDirtyRect);
   }
 }
 

layout/painting/nsDisplayList.h

@@ -1211,13 +1211,16 @@ public:
 
   struct OutOfFlowDisplayData {
     OutOfFlowDisplayData(const DisplayItemClipChain* aContainingBlockClipChain,
+                         const DisplayItemClipChain* aCombinedClipChain,
                          const ActiveScrolledRoot* aContainingBlockActiveScrolledRoot,
                          const nsRect &aDirtyRect)
       : mContainingBlockClipChain(aContainingBlockClipChain)
+      , mCombinedClipChain(aCombinedClipChain)
       , mContainingBlockActiveScrolledRoot(aContainingBlockActiveScrolledRoot)
       , mDirtyRect(aDirtyRect)
     {}
     const DisplayItemClipChain* mContainingBlockClipChain;
+    const DisplayItemClipChain* mCombinedClipChain; // only necessary for the special case of top layer
     const ActiveScrolledRoot* mContainingBlockActiveScrolledRoot;
     nsRect mDirtyRect;
   };