Fix Compositing of Opaque Scrolling Layers and Add Tests

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

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "core/layout/compositing/CompositingReasonFinder.h"
 
 #include "core/CSSPropertyNames.h"
 #include "core/dom/Document.h"
 #include "core/frame/FrameView.h"
 #include "core/frame/Settings.h"
@@ skipping 124 matching lines @@
     // Note that we ask the layoutObject if it has a transform, because the style may have transforms,
     // but the layoutObject may be an inline that doesn't support them.
     return layoutObject->hasTransformRelatedProperty() && layoutObject->style()->has3DTransform();
 }
 
 CompositingReasons CompositingReasonFinder::nonStyleDeterminedDirectReasons(const PaintLayer* layer) const
 {
     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())
             directReasons |= CompositingReasonOverflowScrollingParent;
     }
 
     if (requiresCompositingForPositionFixed(layer))
         directReasons |= CompositingReasonPositionFixed;
 
     directReasons |= layoutObject->additionalCompositingReasons();
@@ skipping 13 matching lines @@
 bool CompositingReasonFinder::requiresCompositingForPositionFixed(const PaintLayer* layer) const
 {
     if (!(m_compositingTriggers & ViewportConstrainedPositionedTrigger))
         return false;
     // Don't promote fixed position elements that are descendants of a non-view container, e.g. transformed elements.
     // They will stay fixed wrt the container rather than the enclosing frame.
     return layer->scrollsWithViewport() && m_layoutView.frameView()->isScrollable();
 }
 
 } // namespace blink