Disable clipping on root scroller's ancestors.

third_party/WebKit/Source/core/layout/compositing/CompositedLayerMapping.cpp

 /*
  * Copyright (C) 2009, 2010, 2011 Apple Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 575 matching lines @@
         updatePaintingPhases();
 
     updateElementIdAndCompositorMutableProperties();
 
     m_graphicsLayer->setHasWillChangeTransformHint(style.hasWillChangeTransformHint());
 
     m_owningLayer.update3DTransformedDescendantStatus();
     if (style.preserves3D() && style.hasOpacity() && m_owningLayer.has3DTransformedDescendant())
         UseCounter::count(layoutObject->document(), UseCounter::OpacityWithPreserve3DQuirk);
 
+    setClippingOnRootScrollerAncestors();
+
     return layerConfigChanged;
 }
 
+void CompositedLayerMapping::setClippingOnRootScrollerAncestors()
+{
+    bool shouldClip = !m_owningLayer.hasRootScrollerAsDescendant();
+    if (m_scrollingLayer)

[chrishtr, 2016/09/01 23:42:56]

Could you try moving these to the sites where we call setMasksToBounds on each
of these layers, and
generalize the logic there?

[bokan, 2016/09/09 00:19:02]

Moved the m_scrollingLayer->setMasksBounds to happen in
CLM::updateScrollingLayers.

+        m_scrollingLayer->setMasksToBounds(shouldClip);
+    if (m_ancestorClippingLayer)

[chrishtr, 2016/09/01 23:42:56]

Why have the m_ancestorClippingLayer layer at all then in this case?

[bokan, 2016/09/09 00:19:02]

Good point, I've moved these to the arguments that get passed to
updateClippingLayers to prevent/remove them in this case.

+        m_ancestorClippingLayer->setMasksToBounds(shouldClip);
+    if (m_childContainmentLayer)

[chrishtr, 2016/09/01 23:42:56]

ditto

[bokan, 2016/09/09 00:19:02]

Done.

+        m_childContainmentLayer->setMasksToBounds(shouldClip);
+}
+
 static IntRect clipBox(LayoutBox* layoutObject)
 {
     // TODO(chrishtr): pixel snapping is most likely incorrect here.
     return pixelSnappedIntRect(layoutObject->clippingRect());
 }
 
 static LayoutPoint computeOffsetFromCompositedAncestor(const PaintLayer* layer, const PaintLayer* compositedAncestor)
 {
     LayoutPoint offset = layer->visualOffsetFromAncestor(compositedAncestor);
     if (compositedAncestor)
@@ skipping 2094 matching lines @@
     } else if (graphicsLayer == m_scrollingContentsLayer.get()) {
         name = "Scrolling Contents Layer";
     } else {
         ASSERT_NOT_REACHED();
     }
 
     return name;
 }
 
 } // namespace blink