Should not repaint if a CSS property change can be composited

Source/core/rendering/RenderLayerModelObject.cpp

Index: Source/core/rendering/RenderLayerModelObject.cpp
diff --git a/Source/core/rendering/RenderLayerModelObject.cpp b/Source/core/rendering/RenderLayerModelObject.cpp
index 82b423b6d1e9e948e10fed043cf89f8d26c50929..ba76efc4e46b45760408a5f60aadffa31c9b9b46 100644
--- a/Source/core/rendering/RenderLayerModelObject.cpp
+++ b/Source/core/rendering/RenderLayerModelObject.cpp
@@ -111,16 +111,21 @@ void RenderLayerModelObject::styleWillChange(StyleDifference diff, const RenderS
             // 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.