Don't always repaint on zIndex change

Source/core/rendering/style/RenderStyle.cpp

Index: Source/core/rendering/style/RenderStyle.cpp
diff --git a/Source/core/rendering/style/RenderStyle.cpp b/Source/core/rendering/style/RenderStyle.cpp
index 8accb94d73609b0186a0da53fcf963a1286545f0..3db5b5d2f7e975148cf920a7930111634f32a388 100644
--- a/Source/core/rendering/style/RenderStyle.cpp
+++ b/Source/core/rendering/style/RenderStyle.cpp
@@ -600,9 +600,12 @@ StyleDifference RenderStyle::visualInvalidationDiff(const RenderStyle& other, un
 
 StyleDifference RenderStyle::repaintOnlyDiff(const RenderStyle& other, unsigned& changedContextSensitiveProperties) const
 {
-    if (position() != StaticPosition && (m_box->zIndex() != other.m_box->zIndex() || m_box->hasAutoZIndex() != other.m_box->hasAutoZIndex()
-        || visual->clip != other.visual->clip || visual->hasClip != other.visual->hasClip))
-        return StyleDifferenceRepaintLayer;
+    if (position() != StaticPosition) {

[ojan, 2014/04/18 01:46:23]

This isn't the fault of your patch, but now that I look at this code, it isn't
quite right. There are other conditions where zIndex applies than just the
position. See the following code in StyleAdjuster::adjustRenderStyle:
    if (style->position() == StaticPosition &&
!parentStyleForcesZIndexToCreateStackingContext(parentStyle))
        style->setHasAutoZIndex();

I don't think fixing this is very high priority, but mind putting a FIXME in
this code while you're here?

[Xianzhu, 2014/04/18 16:05:41]

My understanding about the code is that it forces autoZIndex when non-auto
zIndex is not applicable. Auto noticed that in other cases zIndex will be forced
to 0 in StyleAdjuster.cpp. So StyleAdjuster seems to already handle the
applicable conditions of ZIndex. I'm wondering if it'd be correct to just remove
the condition at line 603 for zIndex.

[ojan, 2014/04/18 17:20:03]

I think you're right. In either case, that should be a separate patch with a
test-case to go with it. So, a FIXME for now?

+        if (m_box->zIndex() != other.m_box->zIndex() || m_box->hasAutoZIndex() != other.m_box->hasAutoZIndex())
+            changedContextSensitiveProperties |= ContextSensitivePropertyZIndex;
+        if (visual->clip != other.visual->clip || visual->hasClip != other.visual->hasClip)
+            return StyleDifferenceRepaintLayer;
+    }
 
     if (RuntimeEnabledFeatures::cssCompositingEnabled() && (rareNonInheritedData->m_effectiveBlendMode != other.rareNonInheritedData->m_effectiveBlendMode
         || rareNonInheritedData->m_isolation != other.rareNonInheritedData->m_isolation))