Main-thread scroll if we have fixed-pos children and overflow:hidden.

Source/core/frame/FrameView.h

Index: Source/core/frame/FrameView.h
diff --git a/Source/core/frame/FrameView.h b/Source/core/frame/FrameView.h
index f2e56f68f001f9a35704c170439738442bf3cef0..b138c80a0a26aab03adbc97e84e448dc96ff3e35 100644
--- a/Source/core/frame/FrameView.h
+++ b/Source/core/frame/FrameView.h
@@ -246,6 +246,14 @@ public:
 
     bool isFrameViewScrollCorner(RenderScrollbarPart* scrollCorner) const { return m_scrollCorner == scrollCorner; }
 
+    enum ScrollingReasons {
+        Scrollable,
+        NonScrollableFromSize,
+        NonScrollableFromVisibility,
+        NonScrollableFromLayout
+    };
+
+    ScrollingReasons scrollingReasons();
     bool isScrollable();
 
     enum ScrollbarModesCalculationStrategy { RulesFromWebContentOnly, AnyRule };