Before finalizing scroll dimensions, check again whether we need scrollbars

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@web.de>
  *   Randall Jesup <rjesup@wgate.com>
@@ skipping 598 matching lines @@
     bool hasHorizontalOverflow = this->hasHorizontalOverflow();
     bool hasVerticalOverflow = this->hasVerticalOverflow();
     if (hasOverlayScrollbars()) {
         if (!scrollSize(HorizontalScrollbar))
             setHasHorizontalScrollbar(false);
         if (!scrollSize(VerticalScrollbar))
             setHasVerticalScrollbar(false);
     }
 
     // overflow:auto may need to lay out again if scrollbars got added/removed.
-    autoHorizontalScrollBarChanged = (box().hasAutoHorizontalScrollbar() && (hasHorizontalScrollbar() != hasHorizontalOverflow)) || (box().style()->overflowX() == OSCROLL && !horizontalScrollbar());
-    autoVerticalScrollBarChanged = (box().hasAutoVerticalScrollbar() && (hasVerticalScrollbar() != hasVerticalOverflow)) || (box().style()->overflowY() == OSCROLL && !verticalScrollbar());
+    autoHorizontalScrollBarChanged |= (box().hasAutoHorizontalScrollbar() && (hasHorizontalScrollbar() != hasHorizontalOverflow)) || (box().style()->overflowX() == OSCROLL && !horizontalScrollbar());
+    autoVerticalScrollBarChanged |= (box().hasAutoVerticalScrollbar() && (hasVerticalScrollbar() != hasVerticalOverflow)) || (box().style()->overflowY() == OSCROLL && !verticalScrollbar());
     if (!visualViewportSuppliesScrollbars() && (autoHorizontalScrollBarChanged || autoVerticalScrollBarChanged)) {
         if (box().hasAutoHorizontalScrollbar())
             setHasHorizontalScrollbar(hasHorizontalOverflow);
         else if (box().style()->overflowX() == OSCROLL)
             setHasHorizontalScrollbar(true);
         if (box().hasAutoVerticalScrollbar())
             setHasVerticalScrollbar(hasVerticalOverflow);
         else if (box().style()->overflowY() == OSCROLL)
             setHasVerticalScrollbar(true);
     }
@@ skipping 83 matching lines @@
     bool hasOverflow = hasScrollableHorizontalOverflow() || hasScrollableVerticalOverflow();
     updateScrollableAreaSet(hasOverflow);
 
     DisableCompositingQueryAsserts disabler;
     positionOverflowControls();
 }
 
 void PaintLayerScrollableArea::updateAfterLayout()
 {
     DoubleSize originalScrollOffset;
-    bool autoHorizontalScrollBarChanged;
-    bool autoVerticalScrollBarChanged;
+    bool autoHorizontalScrollBarChanged = false;
+    bool autoVerticalScrollBarChanged = false;
     updateScrollDimensions(originalScrollOffset, autoHorizontalScrollBarChanged, autoVerticalScrollBarChanged);
     finalizeScrollDimensions(originalScrollOffset, autoHorizontalScrollBarChanged, autoVerticalScrollBarChanged);
 }
 
 ScrollBehavior PaintLayerScrollableArea::scrollBehaviorStyle() const
 {
     return box().style()->scrollBehavior();
 }
 
 bool PaintLayerScrollableArea::hasHorizontalOverflow() const
@@ skipping 790 matching lines @@
 }
 
 DEFINE_TRACE(PaintLayerScrollableArea::ScrollbarManager)
 {
     visitor->trace(m_scrollableArea);
     visitor->trace(m_hBar);
     visitor->trace(m_vBar);
 }
 
 } // namespace blink