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, 2016/08/26 18:24:23]

I don't understand why hasOverflowScrollTrigger() isn't sufficient. Isn't that
still true whether the flag is on or off?

[Stephen Chennney, 2016/08/26 18:42:08]

hasOverflowScrollTrigger only checks for the OverflowScrollTrigger enum and that
is only set based on the preferCompositingToLCDText setting. 

I don't want to mess with hasOverflowScrollTrigger() itself because
PaintLayerCompositor uses it as a proxy for preferCompositingToLCDText. We don't
want to mix up preferCompositingToLCDText and compositeOpaqueScrollers because
the former causes a lot more stuff to get composited, and we don't want some
parts of the system thinking things are composited and others not (that's why
gmail was broken for me before this change).

         if (layer->clipParent())
             directReasons |= CompositingReasonOutOfFlowClipping;