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

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

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  *           (C) 2005 Allan Sandfeld Jensen (kde@carewolf.com)
  *           (C) 2005, 2006 Samuel Weinig (sam.weinig@gmail.com)
  * Copyright (C) 2005, 2006, 2007, 2008, 2009, 2010 Apple Inc.
  *               All rights reserved.
  * Copyright (C) 2013 Adobe Systems Incorporated. All rights reserved.
  *
  * This library is free software; you can redistribute it and/or
@@ skipping 955 matching lines @@
   return GetScrollableArea()->VerticalScrollbarWidth();
 }
 
 int LayoutBox::HorizontalScrollbarHeight() const {
   if (!HasOverflowClip() || Style()->OverflowX() == EOverflow::kOverlay)
     return 0;
 
   return GetScrollableArea()->HorizontalScrollbarHeight();
 }
 
+LayoutUnit LayoutBox::VerticalScrollbarWidthClampedToContentBox() const {
+  LayoutUnit width(VerticalScrollbarWidth());
+  DCHECK_GE(width, LayoutUnit());
+  if (width) {
+    LayoutUnit minimum_width = LogicalWidth() - BorderAndPaddingLogicalWidth();
+    DCHECK_GE(minimum_width, LayoutUnit());
+    width = std::min(width, minimum_width);
+  }
+  return width;
+}
+
 ScrollResult LayoutBox::Scroll(ScrollGranularity granularity,
                                const FloatSize& delta) {
   // Presumably the same issue as in setScrollTop. See crbug.com/343132.
   DisableCompositingQueryAsserts disabler;
 
   if (!GetScrollableArea())
     return ScrollResult();
 
   return GetScrollableArea()->UserScroll(granularity, delta);
 }
@@ skipping 4878 matching lines @@
 void LayoutBox::MutableForPainting::
     SavePreviousContentBoxSizeAndLayoutOverflowRect() {
   auto& rare_data = GetLayoutBox().EnsureRareData();
   rare_data.has_previous_content_box_size_and_layout_overflow_rect_ = true;
   rare_data.previous_content_box_size_ = GetLayoutBox().ContentBoxRect().Size();
   rare_data.previous_layout_overflow_rect_ =
       GetLayoutBox().LayoutOverflowRect();
 }
 
 }  // namespace blink