Get rid of spurious horizontal scrollbars on column flexboxes.

third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.cpp

 /*
  * Copyright (C) 2006, 2007, 2008, 2009, 2010, 2011, 2012 Apple Inc. All rights reserved.
  *
  * Portions are Copyright (C) 1998 Netscape Communications Corporation.
  *
  * Other contributors:
  *   Robert O'Callahan <roc+@cs.cmu.edu>
  *   David Baron <dbaron@fas.harvard.edu>
  *   Christian Biesinger <cbiesinger@gmail.com>
  *   Randall Jesup <rjesup@wgate.com>
@@ skipping 757 matching lines @@
     m_scrollbarManager.destroyDetachedScrollbars();
 }
 
 ScrollBehavior PaintLayerScrollableArea::scrollBehaviorStyle() const
 {
     return box().style()->getScrollBehavior();
 }
 
 bool PaintLayerScrollableArea::hasHorizontalOverflow() const
 {
-    return pixelSnappedScrollWidth() > box().pixelSnappedClientWidth();
+    // TODO(szager): Make the algorithm for adding/subtracting overflow:auto
+    // scrollbars memoryless (crbug.com/625300).  This clientWidth hack will
+    // prevent the spurious horizontal scrollbar, but it can cause a converse
+    // problem: it can leave a sliver of horizontal overflow hidden behind the
+    // vertical scrollbar without creating a horizontal scrollbar.  This
+    // converse problem seems to happen much less frequently in practice, so we
+    // bias the logic towards preventing unwanted horizontal scrollbars, which
+    // are more common and annoying.
+    int clientWidth = box().pixelSnappedClientWidth();
+    if (needsRelayout() && !hadVerticalScrollbarBeforeRelayout())
+        clientWidth += verticalScrollbarWidth();
+    return pixelSnappedScrollWidth() > clientWidth;
 }
 
 bool PaintLayerScrollableArea::hasVerticalOverflow() const
 {
     return pixelSnappedScrollHeight() > box().pixelSnappedClientHeight();
 }
 
 bool PaintLayerScrollableArea::hasScrollableHorizontalOverflow() const
 {
     return hasHorizontalOverflow() && box().scrollsOverflowX();
@@ skipping 953 matching lines @@
 
 void PaintLayerScrollableArea::DelayScrollPositionClampScope::clampScrollableAreas()
 {
     for (auto& scrollableArea : *s_needsClamp)
         scrollableArea->clampScrollPositionsAfterLayout();
     delete s_needsClamp;
     s_needsClamp = nullptr;
 }
 
 } // namespace blink