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()) {
-        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|.
     // 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))
@@ skipping 16 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