Scale the ALPHA_THRESHOLD filter's region based on the device scale factor.

ui/compositor/layer.h

Index: ui/compositor/layer.h
diff --git a/ui/compositor/layer.h b/ui/compositor/layer.h
index 2b0d9138361dd46eabb0dbe199716c63c2256f3e..378478eb3354c84e89534fa675368f8648a2bb98 100644
--- a/ui/compositor/layer.h
+++ b/ui/compositor/layer.h
@@ -374,6 +374,8 @@ class COMPOSITOR_EXPORT Layer
   void RecomputeDrawsContentAndUVRect();
   void RecomputePosition();
 
+  void RecomputeAlphaShape();

[Wez, 2014/06/08 00:55:54]

Add an explanatory comment?

[garykac, 2014/06/09 21:30:00]

Done.

+
   // Set all filters which got applied to the layer.
   void SetLayerFilters();
 
@@ -435,7 +437,11 @@ class COMPOSITOR_EXPORT Layer
   // Width of the border in pixels, where the scaling is blended.
   int zoom_inset_;
 
-  // Shape of the window.
+  // The alpha shape specifies the area that is visible (and matches the area
+  // that accepts input events).
+  // |raw_alpha_shape_| is the shape as specified and |alpha_shape_| is the
+  // shape after being adjusted by the |device_scale_factor_|.
+  scoped_ptr<SkRegion> raw_alpha_shape_;

[Wez, 2014/06/08 00:55:54]

nit: Would |dips_alpha_shape_| and |device_alpha_shape_| here be clearer;
otherwise you have end up having to explain what the "raw" shape is, as in your
comment. :)

[garykac, 2014/06/09 21:29:59]

I updated raw_ to dips_ and updated the comment.

   scoped_ptr<SkRegion> alpha_shape_;
 
   std::string name_;