Enable compositing for opaque scrolling content on low DPI screens

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 15 matching lines @@
     m_compositingTriggers = 0;
 
     Settings& settings = m_layoutView.document().page()->settings();
     if (settings.preferCompositingToLCDTextEnabled()) {
         m_compositingTriggers |= ScrollableInnerFrameTrigger;
         m_compositingTriggers |= OverflowScrollTrigger;
         m_compositingTriggers |= ViewportConstrainedPositionedTrigger;
     }
 }
 
-bool CompositingReasonFinder::hasOverflowScrollTrigger() const
+bool CompositingReasonFinder::hasOverflowScrollTrigger(const PaintLayer* layer) const
 {
-    return m_compositingTriggers & OverflowScrollTrigger;
+    return m_compositingTriggers & OverflowScrollTrigger
+        || !layer->layoutObject()->style()->visitedDependentColor(CSSPropertyBackgroundColor).hasAlpha();

[chrishtr, 2016/05/25 15:55:22]

You'll also have to ensure that the scroller is positioned at exact integer
offsets and is not rotated, mirroring the LCD
logic in cc.

 }
 
 bool CompositingReasonFinder::isMainFrame() const
 {
     // FIXME: LocalFrame::isMainFrame() is probably better.
     return !m_layoutView.document().ownerElement();
 }
 
 CompositingReasons CompositingReasonFinder::directReasons(const PaintLayer* layer) const
 {
@@ skipping 87 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 (hasOverflowScrollTrigger(layer)) {
         if (layer->clipParent())
             directReasons |= CompositingReasonOutOfFlowClipping;
 
         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.
@@ skipping 22 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