Promote opaque fixed position elements.

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 51777eb381099c656b3338a2eac962f1aff9c8df..615151d5e9189c2b731cfc2ae2fea56edbbd70a2 100644
--- a/third_party/WebKit/Source/core/layout/compositing/CompositingReasonFinder.cpp
+++ b/third_party/WebKit/Source/core/layout/compositing/CompositingReasonFinder.cpp
@@ -168,8 +168,15 @@ bool CompositingReasonFinder::requiresCompositingForAnimation(const ComputedStyl
 
 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())