Should not repaint if a CSS property change can be composited

Source/core/rendering/RenderLayerModelObject.cpp

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  *           (C) 2005 Allan Sandfeld Jensen (kde@carewolf.com)
  *           (C) 2005, 2006 Samuel Weinig (sam.weinig@gmail.com)
  * Copyright (C) 2005, 2006, 2007, 2008, 2009 Apple Inc. All rights reserved.
  * Copyright (C) 2010, 2012 Google Inc. All rights reserved.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
@@ skipping 93 matching lines @@
                 if (oldStyle->clip() != newStyle->clip())
                     layer()->clipper().clearClipRectsIncludingDescendants();
             } else if (diff == StyleDifferenceRepaint || newStyle->outlineSize() < oldStyle->outlineSize())
                 repaint();
         }
 
         if (diff == StyleDifferenceLayout || diff == StyleDifferenceSimplifiedLayout) {
             // When a layout hint happens, we go ahead and do a repaint of the layer, since the layer could
             // end up being destroyed.
             if (hasLayer()) {
-                if (oldStyle->position() != newStyle->position()
-                    || oldStyle->zIndex() != newStyle->zIndex()
-                    || oldStyle->hasAutoZIndex() != newStyle->hasAutoZIndex()
-                    || !(oldStyle->clip() == newStyle->clip())
-                    || oldStyle->hasClip() != newStyle->hasClip()
-                    || oldStyle->opacity() != newStyle->opacity()
-                    || oldStyle->transform() != newStyle->transform()
-                    || oldStyle->filter() != newStyle->filter()
-                    )
-                layer()->repainter().repaintIncludingDescendants();
+                if (!(oldStyle->clip() == newStyle->clip())

[esprehn, 2014/03/05 00:09:49]

Use != instead

+                    || oldStyle->hasClip() != newStyle->hasClip()) {
+                    // Composited layers don't need to be repainted when
+                    // a parent's clip changes.

[esprehn, 2014/03/05 00:09:49]

Don't wrap the comment.

+                    layer()->repainter().repaintIncludingNonCompositingDescendants(this);
+                } else if (!layer()->hasCompositedLayerMapping()) {

[esprehn, 2014/03/05 00:09:49]

How do we know this is safe? The other properties could have changed too, like
hasAutoZIndex or position.

It's not clear why when clip changes and lots of other things do too we can take
this special branch. Shouldn't we check the other stuff first?

+                    if (oldStyle->position() != newStyle->position()
+                        || oldStyle->zIndex() != newStyle->zIndex()
+                        || oldStyle->hasAutoZIndex() != newStyle->hasAutoZIndex()
+                        || oldStyle->opacity() != newStyle->opacity()
+                        || oldStyle->transform() != newStyle->transform()
+                        || oldStyle->filter() != newStyle->filter()
+                        )

[esprehn, 2014/03/05 00:09:49]

paren should be in the previous line

+                    layer()->repainter().repaintIncludingDescendants();
+                }
             } else if (newStyle->hasTransform() || newStyle->opacity() < 1 || newStyle->hasFilter()) {
                 // If we don't have a layer yet, but we are going to get one because of transform or opacity,
                 //  then we need to repaint the old position of the object.
                 repaint();
             }
         }
     }
 
     RenderObject::styleWillChange(diff, newStyle);
 }
@@ skipping 80 matching lines @@
     return m_layer ? m_layer->hasCompositedLayerMapping() : false;
 }
 
 CompositedLayerMapping* RenderLayerModelObject::groupedMapping() const
 {
     return m_layer ? m_layer->groupedMapping() : 0;
 }
 
 } // namespace WebCore