Promote fixed: position elements with composited descendants even if they don't scroll

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 176 matching lines @@
     // FIXME: This should move into
     // CompositingReasonFinder::potentialCompositingReasonsFromStyle, but theres
     // a poor interaction with LayoutTextControlSingleLine, which sets this
     // hasOverflowClip directly.
     if (layer->GetLayoutObject().HasClipRelatedProperty())
       subtree_reasons |= kCompositingReasonClipsCompositingDescendants;
 
     // We ignore LCD text here because we are required to composite
     // scroll-dependant fixed position elements with composited descendants for
     // correctness - even if we lose LCD.
+    //
+    // TODO(smcgruer): Only composite fixed if needed (http://crbug.com/742213)
     const bool ignore_lcd_text = true;
-    if (compositing_reason_finder.RequiresCompositingForScrollDependentPosition(
+    if (layer->GetLayoutObject().Style()->GetPosition() == EPosition::kFixed ||
+        compositing_reason_finder.RequiresCompositingForScrollDependentPosition(
             layer, ignore_lcd_text)) {
       subtree_reasons |=
           kCompositingReasonPositionFixedOrStickyWithCompositedDescendants;
     }
   }
 
   // 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 (has3d_transformed_descendants) {
@@ skipping 357 matching lines @@
     descendant_has3d_transform |=
         any_descendant_has3d_transform || layer->Has3DTransform();
   }
 
   // At this point we have finished collecting all reasons to composite this
   // layer.
   layer->SetCompositingReasons(reasons_to_composite);
 }
 
 }  // namespace blink