Disable scrollbars on the root scroller when using visual viewport scrollbars.

third_party/WebKit/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
@@ skipping 751 matching lines @@
   bool scrollContentsFastPath(const IntSize& scrollDelta);
 
   // Scroll the content by invalidating everything.
   void scrollContentsSlowPath();
 
   ScrollBehavior scrollBehaviorStyle() const override;
 
   void scrollContentsIfNeeded();
 
   enum ComputeScrollbarExistenceOption { FirstPass, Incremental };
-  void computeScrollbarExistence(
-      bool& newHasHorizontalScrollbar,
-      bool& newHasVerticalScrollbar,
-      const IntSize& docSize,
-      ComputeScrollbarExistenceOption = FirstPass) const;
+  void computeScrollbarExistence(bool& newHasHorizontalScrollbar,
+                                 bool& newHasVerticalScrollbar,
+                                 const IntSize& docSize,
+                                 ComputeScrollbarExistenceOption = FirstPass);
   void updateScrollbarGeometry();
 
   // Called to update the scrollbars to accurately reflect the state of the
   // view.
   void updateScrollbars();
   void updateScrollbarsIfNeeded();
 
   class InUpdateScrollbarsScope {
     STACK_ALLOCATED();
 
@@ skipping 74 matching lines @@
   void contentsResized() override;
   void scrollbarExistenceDidChange();
 
   // Override Widget methods to do point conversion via layoutObjects, in order
   // to take transforms into account.
   IntRect convertToContainingWidget(const IntRect&) const override;
   IntRect convertFromContainingWidget(const IntRect&) const override;
   IntPoint convertToContainingWidget(const IntPoint&) const override;
   IntPoint convertFromContainingWidget(const IntPoint&) const override;
 
+  void didChangeGlobalRootScroller() override;
+
   void updateWidgetGeometriesIfNeeded();
 
   bool wasViewportResized();
   void sendResizeEventIfNeeded();
 
   void updateParentScrollableAreaSet();
 
   void scheduleUpdateWidgetsIfNecessary();
   void updateWidgetsTimerFired(TimerBase*);
   bool updateWidgets();
@@ skipping 27 matching lines @@
 
   void updateScrollCorner();
 
   AXObjectCache* axObjectCache() const;
 
   void setLayoutSizeInternal(const IntSize&);
 
   bool adjustScrollbarExistence(ComputeScrollbarExistenceOption = FirstPass);
   void adjustScrollbarOpacity();
   void adjustScrollOffsetFromUpdateScrollbars();
-  bool visualViewportSuppliesScrollbars() const;
+  bool visualViewportSuppliesScrollbars();

[skobes, 2016/11/15 20:18:43]

Why can't this be const anymore?

[bokan, 2016/11/15 22:04:10]

Because of the check: root scroller == layoutViewportScrollableArea(),
layoutViewportScrollableArea can't be const because if RLS is disabled it
returns `this`. 

The solution would be to add a const overload of layoutViewportScrollableArea()
but we've decided long ago that the ship has sailed on const-correctness.

 
   bool isFrameViewScrollbar(const Widget* child) const {
     return horizontalScrollbar() == child || verticalScrollbar() == child;
   }
 
   ScrollingCoordinator* scrollingCoordinator() const;
 
   void prepareLayoutAnalyzer();
   std::unique_ptr<TracedValue> analyzerCounters();
 
@@ skipping 221 matching lines @@
                   widget.isFrameView());
 DEFINE_TYPE_CASTS(FrameView,
                   ScrollableArea,
                   scrollableArea,
                   scrollableArea->isFrameView(),
                   scrollableArea.isFrameView());
 
 }  // namespace blink
 
 #endif  // FrameView_h