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..2b72bb7c641c10ce3f590ed6361330168100df87 100644
--- a/third_party/WebKit/Source/core/layout/compositing/CompositingReasonFinder.cpp
+++ b/third_party/WebKit/Source/core/layout/compositing/CompositingReasonFinder.cpp
@@ -142,16 +142,11 @@ CompositingReasons CompositingReasonFinder::nonStyleDeterminedDirectReasons(cons
     CompositingReasons directReasons = CompositingReasonNone;
     LayoutObject* layoutObject = layer->layoutObject();
 
-    if (hasOverflowScrollTrigger()) {

[chrishtr, 2016/09/07 20:38:07]

Should hasOverflowScrollTrigger() now be deleted?
Its other call site is
PaintLayerCompositor::preferCompositingToLCDTextEnabled(),
which could just consult
document().page->settings().preferCompositingToLCDTextEnabled() instead.

[Stephen Chennney, 2016/09/07 20:38:40]

This has been removed because layer->needsCompositedScrolling() takes care of
the CompositingReasonOverflowScrollingTouch reason, while the
CompositingReasonOutOfFlowClipping has been moved to the place it's used in
CompositingRequirementsUpdater.

[Stephen Chennney, 2016/09/07 20:49:36]

Yeah, I don't see why not.

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

[Stephen Chennney, 2016/09/07 20:38:40]

This is only used in CompositingRequirementsUpdater, so move it there

-
-        if (layer->needsCompositedScrolling())
-            directReasons |= CompositingReasonOverflowScrollingTouch;

[Stephen Chennney, 2016/09/07 20:38:40]

This check is all that remains.

-    }
+    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|.
+    // scrolling layer is not on the stacking context ancestor chain of |layer|.
     // See the definition of the scrollParent property in Layer for more detail.
     if (const PaintLayer* scrollingAncestor = layer->ancestorScrollingLayer()) {
         if (scrollingAncestor->needsCompositedScrolling() && layer->scrollParent())