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 ef055cc18964eb27de23fcfc35cacc2efc0dc50e..634cb8a2630430a34db7e24667a72f412afe5aed 100644
--- a/Source/core/rendering/style/RenderStyle.cpp
+++ b/Source/core/rendering/style/RenderStyle.cpp
@@ -605,9 +605,14 @@ StyleDifferenceLegacy RenderStyle::visualInvalidationDiffLegacy(const RenderStyl
 
 StyleDifferenceLegacy 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;
+    // FIXME: The condition for zIndex may be incorrect or unnecessary.
+    // See StyleAdjuster::adjustRenderStyle() for the conditions that zIndex is applicable.
+    if (position() != StaticPosition) {
+        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))