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

Source/core/frame/FrameView.h

 /*
    Copyright (C) 1997 Martin Jones (mjones@kde.org)
              (C) 1998 Waldo Bastian (bastian@kde.org)
              (C) 1998, 1999 Torben Weis (weis@kde.org)
              (C) 1999 Lars Knoll (knoll@kde.org)
              (C) 1999 Antti Koivisto (koivisto@kde.org)
    Copyright (C) 2004, 2005, 2006, 2007, 2008, 2009 Apple 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 228 matching lines @@
     void scrollContentsIfNeededRecursive();
 
     // Methods to convert points and rects between the coordinate space of the renderer, and this view.
     IntRect convertFromRenderer(const RenderObject&, const IntRect&) const;
     IntRect convertToRenderer(const RenderObject&, const IntRect&) const;
     IntPoint convertFromRenderer(const RenderObject&, const IntPoint&) const;
     IntPoint convertToRenderer(const RenderObject&, const IntPoint&) const;
 
     bool isFrameViewScrollCorner(RenderScrollbarPart* scrollCorner) const { return m_scrollCorner == scrollCorner; }
 
+    enum ScrollingReasons {
+        Scrollable,
+        NotScrollableNoOverflow,
+        NotScrollableNotVisible,
+        NotScrollableExplicitlyDisabled
+    };
+
+    ScrollingReasons scrollingReasons();
     bool isScrollable();
 
     enum ScrollbarModesCalculationStrategy { RulesFromWebContentOnly, AnyRule };
     void calculateScrollbarModesForLayoutAndSetViewportRenderer(ScrollbarMode& hMode, ScrollbarMode& vMode, ScrollbarModesCalculationStrategy = AnyRule);
 
     virtual IntPoint lastKnownMousePosition() const override;
     bool shouldSetCursor() const;
 
     void setCursor(const Cursor&);
 
@@ skipping 568 matching lines @@
     }
 
 private:
     RawPtrWillBeMember<FrameView> m_view;
     bool m_originalValue;
 };
 
 } // namespace blink
 
 #endif // FrameView_h