Disable clipping on root scroller's ancestors.

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

Index: third_party/WebKit/Source/core/layout/compositing/CompositedLayerMapping.cpp
diff --git a/third_party/WebKit/Source/core/layout/compositing/CompositedLayerMapping.cpp b/third_party/WebKit/Source/core/layout/compositing/CompositedLayerMapping.cpp
index 72cdc71f262586e46fe0df356fcf1e8ab8d21d7b..13bcef09fe649d7d8265b0b7a8a748e715264a5f 100644
--- a/third_party/WebKit/Source/core/layout/compositing/CompositedLayerMapping.cpp
+++ b/third_party/WebKit/Source/core/layout/compositing/CompositedLayerMapping.cpp
@@ -593,9 +593,22 @@ bool CompositedLayerMapping::updateGraphicsLayerConfiguration()
     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.