When moving past a left-hand scrollbar, don't jump way outside the content box.

third_party/WebKit/Source/core/layout/LayoutBox.h

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  * Copyright (C) 2003, 2006, 2007 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
  * License as published by the Free Software Foundation; either
  * version 2 of the License, or (at your option) any later version.
  *
@@ skipping 997 matching lines @@
   int VerticalScrollbarWidth() const;
   int HorizontalScrollbarHeight() const;
   int ScrollbarLogicalWidth() const {
     return Style()->IsHorizontalWritingMode() ? VerticalScrollbarWidth()
                                               : HorizontalScrollbarHeight();
   }
   int ScrollbarLogicalHeight() const {
     return Style()->IsHorizontalWritingMode() ? HorizontalScrollbarHeight()
                                               : VerticalScrollbarWidth();
   }
+
+  // Return the width of the vertical scrollbar, unless it's larger than the
+  // logical width of the content box, in which case we'll return that instead.
+  // Scrollbar handling is quite bad in such situations, and this method here
+  // is just to make sure that left-hand scrollbars don't mess up
+  // scrollWidth. For the full story, visit crbug.com/724255
+  LayoutUnit VerticalScrollbarWidthClampedToContentBox() const;
+
   virtual ScrollResult Scroll(ScrollGranularity, const FloatSize&);
   bool CanBeScrolledAndHasScrollableArea() const;
   virtual bool CanBeProgramaticallyScrolled() const;
   virtual void Autoscroll(const IntPoint&);
   bool CanAutoscroll() const;
   IntSize CalculateAutoscrollDirection(
       const IntPoint& point_in_root_frame) const;
   static LayoutBox* FindAutoscrollable(LayoutObject*);
   virtual void StopAutoscroll() {}
 
@@ skipping 708 matching lines @@
          break_value == EBreakBetween::kLeft ||
          break_value == EBreakBetween::kPage ||
          break_value == EBreakBetween::kRecto ||
          break_value == EBreakBetween::kRight ||
          break_value == EBreakBetween::kVerso;
 }
 
 }  // namespace blink
 
 #endif  // LayoutBox_h