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..69855833d23bd67d281b9749f327cf8c77bdcd31 100644
--- a/third_party/WebKit/Source/core/layout/compositing/CompositingReasonFinder.cpp
+++ b/third_party/WebKit/Source/core/layout/compositing/CompositingReasonFinder.cpp
@@ -142,7 +142,7 @@ CompositingReasons CompositingReasonFinder::nonStyleDeterminedDirectReasons(cons
     CompositingReasons directReasons = CompositingReasonNone;
     LayoutObject* layoutObject = layer->layoutObject();
 
-    if (hasOverflowScrollTrigger()) {
+    if (hasOverflowScrollTrigger() || RuntimeEnabledFeatures::compositeOpaqueScrollersEnabled()) {

[flackr, 2016/08/30 21:14:49]

You mentioned that hasOverflowScrollTrigger() basically proxies whether we
prefer compositing to lcd text, but it seems like layerNeedsCompositedScrolling
in PaintLayerScrollableArea.cpp already includes whether prefer compositing to
lcd text. Assuming my following comment is not a problem, this condition can
probably be removed altogether.

[Stephen Chennney, 2016/08/31 20:50:20]

I don't think we can remove it for the scrolling case, but adjusting logic will
clean it up and prevent confusion as to what applies where.

         if (layer->clipParent())

[flackr, 2016/08/30 21:14:50]

I'm concerned this condition combined with
RuntimeEnabledFeatures::compositeOpaqueScrollersEnabled() could cause us to add
a compositing reason to a scroller when it doesn't have an opaque background. We
should test this.

[Stephen Chennney, 2016/08/31 20:50:20]

I think I'll just adjust the logic.

[flackr, 2016/09/01 21:06:46]

Sorry for the delay, I mistook this to mean you were going to have another
comment later when this was done.

             directReasons |= CompositingReasonOutOfFlowClipping;