Bug 1272525 - Set a layer clip rather than a scroll clip for fixed background layers if not using a widget layer manager. r=mstange

MozReview-Commit-ID: 2fJyQXSLzsU

--HG--
extra : source : 8a282bae4fa3451efb3187d695ce0d523262acfd
extra : amend_source : 2e3d55a4cee03f6c3c58fe85e98f87a034f2bd39
extra : histedit_source : f6d5b5bfa98795824c0968065011b9ff4a4a6f5d

layout/base/FrameLayerBuilder.cpp

@@ -4116,9 +4116,12 @@ ContainerState::ProcessDisplayItems(nsDisplayList* aList)
       ownLayer->SetClipRect(Nothing());
       ownLayer->SetScrolledClip(Nothing());
       if (layerClip.HasClip()) {
-        if (shouldFixToViewport) {
-          // For layers fixed to the viewport, the clip becomes part of the
-          // layer's scrolled clip.
+        // For layers fixed to the viewport, the clip becomes part of the
+        // layer's scrolled clip. However, BasicLayerManager doesn't support
+        // 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()) {
           LayerClip scrolledClip;
           scrolledClip.SetClipRect(layerClipRect);
           if (layerClip.IsRectClippedByRoundedCorner(itemContent)) {