[WIP] Test moving stacking context flag update on ComputedStyle

third_party/WebKit/Source/core/css/resolver/StyleAdjuster.cpp

Index: third_party/WebKit/Source/core/css/resolver/StyleAdjuster.cpp
diff --git a/third_party/WebKit/Source/core/css/resolver/StyleAdjuster.cpp b/third_party/WebKit/Source/core/css/resolver/StyleAdjuster.cpp
index dc21e25edac0fcf4d09c08933395faf70e5057f6..aba8fa8634622d08f870e7d8828aa722c48d9fe8 100644
--- a/third_party/WebKit/Source/core/css/resolver/StyleAdjuster.cpp
+++ b/third_party/WebKit/Source/core/css/resolver/StyleAdjuster.cpp
@@ -448,6 +448,10 @@ void StyleAdjuster::adjustComputedStyle(ComputedStyle& style,
     style.setIsStackingContext(true);
   }
 
+  style.updateIsStackingContext(
+      element == element->document().documentElement(),
+      element->isInTopLayer());
+
   if (style.overflowX() != EOverflow::kVisible ||
       style.overflowY() != EOverflow::kVisible)
     adjustOverflow(style);