Fix Compositing of Opaque Scrolling Layers and Add Tests

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

Index: third_party/WebKit/Source/core/layout/compositing/CompositingReasonFinder.cpp
diff --git a/third_party/WebKit/Source/core/layout/compositing/CompositingReasonFinder.cpp b/third_party/WebKit/Source/core/layout/compositing/CompositingReasonFinder.cpp
index a4a6beda01478803728ea06224b4ddd4f7e0ce07..fc4af01767c05f23b61e92983ae16ec40989b61e 100644
--- a/third_party/WebKit/Source/core/layout/compositing/CompositingReasonFinder.cpp
+++ b/third_party/WebKit/Source/core/layout/compositing/CompositingReasonFinder.cpp
@@ -142,13 +142,11 @@ CompositingReasons CompositingReasonFinder::nonStyleDeterminedDirectReasons(cons
     CompositingReasons directReasons = CompositingReasonNone;
     LayoutObject* layoutObject = layer->layoutObject();
 
-    if (hasOverflowScrollTrigger()) {
-        if (layer->clipParent())
-            directReasons |= CompositingReasonOutOfFlowClipping;
+    if (hasOverflowScrollTrigger() && layer->clipParent())
+        directReasons |= CompositingReasonOutOfFlowClipping;

[chrishtr, 2016/09/02 01:19:38]

Why doesn't this apply to low-DPI composited scrolling?

[Stephen Chennney, 2016/09/07 19:57:54]

It presumably should, but note that flackr had concerns about adding this reason
before we knew the layer would actually be composited, hence this refactoring.

In the forthcoming patch I've looked at how the
CompositingReasonOutOfFlowClipping reason is used and modified the single use
location to add the reason directly, since it's in
CompositingRequirementsUpdater and that's about the only place we have the
necessary information to apply it correctly.

 
-        if (layer->needsCompositedScrolling())
-            directReasons |= CompositingReasonOverflowScrollingTouch;
-    }
+    if (layer->needsCompositedScrolling())
+        directReasons |= CompositingReasonOverflowScrollingTouch;
 
     // Composite |layer| if it is inside of an ancestor scrolling layer, but that
     // scrolling layer is not not on the stacking context ancestor chain of |layer|.