Don't always repaint on zIndex change

Source/core/rendering/RenderObject.cpp

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  *           (C) 2000 Dirk Mueller (mueller@kde.org)
  *           (C) 2004 Allan Sandfeld Jensen (kde@carewolf.com)
  * Copyright (C) 2004, 2005, 2006, 2007, 2008, 2011 Apple Inc. All rights reserved.
  * Copyright (C) 2009 Google Inc. All rights reserved.
  * Copyright (C) 2009 Torch Mobile Inc. All rights reserved. (http://www.torchmobile.com/)
  *
  * This library is free software; you can redistribute it and/or
@@ skipping 1885 matching lines @@
             // then we actually need SimplifiedLayoutAndPositionedMovement.
             if (!hasLayer())
                 diff.setNeedsFullLayout(); // FIXME: Do this for now since SimplifiedLayout cannot handle updating floating objects lists.
             else
                 diff.setNeedsSimplifiedLayout();
         } else {
             diff.setNeedsRecompositeLayer();
         }
     }
 
-    // If opacity or filters changed, and the layer does not paint into its own separate backing, then we need to repaint (also
+    // If opacity or zIndex changed, and the layer does not paint into its own separate backing, then we need to repaint (also
     // ignoring text nodes)
-    if (contextSensitiveProperties & ContextSensitivePropertyOpacity && !diff.needsLayout()) {
+    if ((contextSensitiveProperties & (ContextSensitivePropertyOpacity | ContextSensitivePropertyZIndex)) && !diff.needsLayout()) {
         if (!isText() && (!hasLayer() || !toRenderLayerModelObject(this)->layer()->hasDirectReasonsForCompositing()))
             diff.setNeedsRepaintLayer();
         else
             diff.setNeedsRecompositeLayer();
     }
 
+    // If filter changed, and the layer does not paint into its own separate backing or it paints with filters, then we need to repaint.
     if ((contextSensitiveProperties & ContextSensitivePropertyFilter) && hasLayer() && !diff.needsLayout()) {
         RenderLayer* layer = toRenderLayerModelObject(this)->layer();
         if (!layer->hasDirectReasonsForCompositing() || layer->paintsWithFilters())
             diff.setNeedsRepaintLayer();
         else
             diff.setNeedsRecompositeLayer();
     }
 
     if ((contextSensitiveProperties & ContextSensitivePropertyTextOrColor) && !diff.needsRepaint() && !diff.needsLayout()
         && hasImmediateNonWhitespaceTextChildOrPropertiesDependentOnColor())
@@ skipping 1493 matching lines @@
 {
     if (object1) {
         const WebCore::RenderObject* root = object1;
         while (root->parent())
             root = root->parent();
         root->showRenderTreeAndMark(object1, "*", object2, "-", 0);
     }
 }
 
 #endif