Bug 1273250 - Set a layer clip rather than a scroll clip for fixed background layers if the clip moves with the layer. r=mstange

This patch also removes the earlier workaround added in bug 1272525.

MozReview-Commit-ID: HR2wF2BGsKl

--HG--
extra : rebase_source : b561f8721d91c930bced7664f11a331552b97a5e
extra : histedit_source : d57e430f76851ce252416aa77fdaad0897d03756

layout/base/FrameLayerBuilder.cpp

@@ -3880,7 +3880,10 @@ ContainerState::ProcessDisplayItems(nsDisplayList* aList)
       item->SetClip(mBuilder, clip);
     }
 
-    bool shouldFixToViewport = !(*animatedGeometryRoot)->GetParent() &&
+    bool clipMovesWithLayer = (animatedGeometryRoot == animatedGeometryRootForClip);
+
+    bool shouldFixToViewport = !clipMovesWithLayer &&
+        !(*animatedGeometryRoot)->GetParent() &&
         item->ShouldFixToViewport(mBuilder);
 
     // For items that are fixed to the viewport, remove their clip at the
@@ -4120,7 +4123,7 @@ ContainerState::ProcessDisplayItems(nsDisplayList* aList)
         // scrolled clips, and doesn't care about the distinction anyways
         // (it only matters for async scrolling), so only set a scrolled clip
         // if we have a widget layer manager.
-        if (shouldFixToViewport && mManager->IsWidgetLayerManager()) {
+        if (shouldFixToViewport) {
           LayerClip scrolledClip;
           scrolledClip.SetClipRect(layerClipRect);
           if (layerClip.IsRectClippedByRoundedCorner(itemContent)) {