[SPInvalidation] Force subtree paint property update on css position change

third_party/WebKit/Source/core/layout/LayoutBoxModelObject.cpp

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  *           (C) 2005 Allan Sandfeld Jensen (kde@carewolf.com)
  *           (C) 2005, 2006 Samuel Weinig (sam.weinig@gmail.com)
  * Copyright (C) 2005, 2006, 2007, 2008, 2009 Apple Inc. All rights reserved.
  * Copyright (C) 2010 Google Inc. All rights reserved.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
@@ skipping 288 matching lines @@
       setNeedsLayoutAndPrefWidthsRecalcAndFullPaintInvalidation(
           LayoutInvalidationReason::StyleChange);
     }
     if (!needsLayout()) {
       // FIXME: We should call a specialized version of this function.
       parentLayer->updateLayerPositionsAfterLayout();
     }
   }
 
   if (RuntimeEnabledFeatures::slimmingPaintInvalidationEnabled()) {
-    // hasLayer status will affect whether to create localBorderBoxProperties.
-    // hasTransformRelatedProperty will affect whether to create transform node.
-    if (hadLayer != hasLayer() ||
-        hadTransformRelatedProperty != hasTransformRelatedProperty()) {
-      setNeedsPaintPropertyUpdate();
-    } else if (oldStyle && oldStyle->position() != styleRef().position() &&
-               (oldStyle->position() == FixedPosition ||
-                styleRef().position() == FixedPosition)) {
-      // Fixed-position status affects whether to create paintOffsetTranslation.
-      // TODO(chrishtr): Update the condition here when changing the condition
-      // in PaintPropertyTreeBuilder::updatePaintOffsetTranslation().
+    if ((oldStyle && oldStyle->position() != styleRef().position()) ||
+        hadLayer != hasLayer()) {
+      // This may affect paint properties of the current object, and descendants
+      // even if paint properties of the current object won't change. E.g. the
+      // stacking context and/or containing block of descendants may change.
+      setSubtreeNeedsPaintPropertyUpdate();
+    } else if (hadTransformRelatedProperty != hasTransformRelatedProperty()) {
+      // This affects whether to create transform node.
       setNeedsPaintPropertyUpdate();
     }
   }
 
   if (layer()) {
     layer()->styleDidChange(diff, oldStyle);
     if (hadLayer && layer()->isSelfPaintingLayer() != layerWasSelfPainting)
       setChildNeedsLayout();
   }
 
@@ skipping 1009 matching lines @@
   if (rootElementStyle->hasBackground())
     return false;
 
   if (node() != document().firstBodyElement())
     return false;
 
   return true;
 }
 
 }  // namespace blink