Don't always repaint on zIndex change

Source/core/rendering/RenderObject.cpp

Index: Source/core/rendering/RenderObject.cpp
diff --git a/Source/core/rendering/RenderObject.cpp b/Source/core/rendering/RenderObject.cpp
index 46d7667427c235faa2e915fc37a35711f5a587cb..281c851dda67bbac5ba304a05fe89bbceaddd0af 100644
--- a/Source/core/rendering/RenderObject.cpp
+++ b/Source/core/rendering/RenderObject.cpp
@@ -1873,15 +1873,16 @@ StyleDifference RenderObject::adjustStyleDifference(StyleDifference diff, unsign
             diff = StyleDifferenceRecompositeLayer;
     }
 
-    // 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 <= StyleDifferenceRepaintLayer) {
+    if ((contextSensitiveProperties & (ContextSensitivePropertyOpacity | ContextSensitivePropertyZIndex)) && diff <= StyleDifferenceRepaintLayer) {
         if (!isText() && (!hasLayer() || !toRenderLayerModelObject(this)->layer()->hasDirectReasonsForCompositing()))
             diff = StyleDifferenceRepaintLayer;
         else if (diff < StyleDifferenceRecompositeLayer)
             diff = StyleDifferenceRecompositeLayer;
     }
 
+    // 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 <= StyleDifferenceRepaintLayer) {
         RenderLayer* layer = toRenderLayerModelObject(this)->layer();
         if (!layer->hasDirectReasonsForCompositing() || layer->paintsWithFilters())