Promote opaque fixed position elements.

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 150 matching lines @@
 bool CompositingReasonFinder::requiresCompositingForAnimation(const ComputedStyle& style) const
 {
     if (style.subtreeWillChangeContents())
         return style.isRunningAnimationOnCompositor();
 
     return style.shouldCompositeForCurrentAnimations();
 }
 
 bool CompositingReasonFinder::requiresCompositingForScrollDependentPosition(const PaintLayer* layer) const
 {
-    if (!(m_compositingTriggers & ViewportConstrainedPositionedTrigger))
+    if (layer->layoutObject()->style()->position() != FixedPosition
+        && layer->layoutObject()->style()->position() != StickyPosition)

[flackr, 2016/09/23 18:19:35]

This is just an early out to prevent doing the work of calling
backgroundIsKnownToBeOpaqueInRect and boundingBoxForCompositing on elements
which will return false.

         return false;
+
+    if (!(m_compositingTriggers & ViewportConstrainedPositionedTrigger)
+        && (!RuntimeEnabledFeatures::compositeOpaqueFixedPositionEnabled()
+            || !layer->backgroundIsKnownToBeOpaqueInRect(LayoutRect(layer->boundingBoxForCompositing())))) {
+        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.
     if (layer->scrollsWithViewport())
         return m_layoutView.frameView()->isScrollable();
     return layer->layoutObject()->style()->position() == StickyPosition && layer->ancestorOverflowLayer()->scrollsOverflow();
 }
 
 } // namespace blink