Always composite position: fixed elements with composited descendants.

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

 /*
  * Copyright (C) 2009, 2010 Apple Inc. All rights reserved.
  * Copyright (C) 2014 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
@@ skipping 150 matching lines @@
 
         if (layer->shouldIsolateCompositedDescendants()) {
             ASSERT(layer->stackingNode()->isStackingContext());
             subtreeReasons |= CompositingReasonIsolateCompositedDescendants;
         }
 
         // FIXME: This should move into CompositingReasonFinder::potentialCompositingReasonsFromStyle, but
         // theres a poor interaction with LayoutTextControlSingleLine, which sets this hasOverflowClip directly.
         if (layer->layoutObject()->hasClipOrOverflowClip())
             subtreeReasons |= CompositingReasonClipsCompositingDescendants;
+
+        if (layer->layoutObject()->style()->position() == FixedPosition)
+            subtreeReasons |= CompositingReasonPositionFixedWithCompositedDescendants;
     }
 
     // A layer with preserve-3d or perspective only needs to be composited if there are descendant layers that
     // will be affected by the preserve-3d or perspective.
     if (has3DTransformedDescendants)
         subtreeReasons |= layer->potentialCompositingReasonsFromStyle() & CompositingReasonCombo3DDescendants;
 
     return subtreeReasons;
 }
 
@@ skipping 264 matching lines @@
 
         descendantHas3DTransform |= anyDescendantHas3DTransform || layer->has3DTransform();
     }
 
     // At this point we have finished collecting all reasons to composite this layer.
     layer->setCompositingReasons(reasonsToComposite);
 
 }
 
 } // namespace blink