Fix invalidation during painting of scrollbars

Source/core/layout/LayerScrollableArea.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 688 matching lines @@
         }
         if (Scrollbar* verticalScrollbar = this->verticalScrollbar()) {
             int clientHeight = box().pixelSnappedClientHeight();
             verticalScrollbar->setProportion(clientHeight, overflowRect().height());
         }
     }
 
     bool hasOverflow = hasScrollableHorizontalOverflow() || hasScrollableVerticalOverflow();
     updateScrollableAreaSet(hasOverflow);
 
-    if (hasOverflow) {
-        DisableCompositingQueryAsserts disabler;
-        positionOverflowControls(IntSize());
-    }
+    DisableCompositingQueryAsserts disabler;
+    positionOverflowControls();

[Julien - ping for review, 2015/02/25 01:43:36]

If we have no overflow it seems wasteful to call positionOverflowControls. Why
do we need to do it?

[Xianzhu, 2015/02/25 01:57:09]

Even if we have no overflow, we can still have scrollbars (e.g. overflow:
scroll). Discovered this when running layout tests. positionOverflowControls()
will be fast if there is no scrollbars.

 }
 
 bool LayerScrollableArea::hasHorizontalOverflow() const
 {
     ASSERT(!m_scrollDimensionsDirty);
 
     return pixelSnappedScrollWidth() > box().pixelSnappedClientWidth();
 }
 
 bool LayerScrollableArea::hasVerticalOverflow() const
@@ skipping 277 matching lines @@
     return m_vBar->width();
 }
 
 int LayerScrollableArea::horizontalScrollbarHeight(OverlayScrollbarSizeRelevancy relevancy) const
 {
     if (!m_hBar || (m_hBar->isOverlayScrollbar() && (relevancy == IgnoreOverlayScrollbarSize || !m_hBar->shouldParticipateInHitTesting())))
         return 0;
     return m_hBar->height();
 }
 
-void LayerScrollableArea::positionOverflowControls(const IntSize& offsetFromRoot)
+void LayerScrollableArea::positionOverflowControls()
 {
     if (!hasScrollbar() && !box().canResize())
         return;
 
     const IntRect borderBox = box().pixelSnappedBorderBoxRect();
-    if (Scrollbar* verticalScrollbar = this->verticalScrollbar()) {
-        IntRect vBarRect = rectForVerticalScrollbar(borderBox);
-        vBarRect.move(offsetFromRoot);
-        verticalScrollbar->setFrameRect(vBarRect);
-    }
+    if (Scrollbar* verticalScrollbar = this->verticalScrollbar())
+        verticalScrollbar->setFrameRect(rectForVerticalScrollbar(borderBox));
 
-    if (Scrollbar* horizontalScrollbar = this->horizontalScrollbar()) {
-        IntRect hBarRect = rectForHorizontalScrollbar(borderBox);
-        hBarRect.move(offsetFromRoot);
-        horizontalScrollbar->setFrameRect(hBarRect);
-    }
+    if (Scrollbar* horizontalScrollbar = this->horizontalScrollbar())
+        horizontalScrollbar->setFrameRect(rectForHorizontalScrollbar(borderBox));
 
     const IntRect& scrollCorner = scrollCornerRect();
     if (m_scrollCorner)
         m_scrollCorner->setFrameRect(scrollCorner);
 
     if (m_resizer)
         m_resizer->setFrameRect(resizerCornerRect(borderBox, ResizerForPointer));
 
     // FIXME, this should eventually be removed, once we are certain that composited
     // controls get correctly positioned on a compositor update. For now, conservatively
     // leaving this unchanged.
     if (layer()->hasCompositedLayerMapping())
-        layer()->compositedLayerMapping()->positionOverflowControlsLayers(offsetFromRoot);
+        layer()->compositedLayerMapping()->positionOverflowControlsLayers();
 }
 
 void LayerScrollableArea::updateScrollCornerStyle()
 {
     if (!m_scrollCorner && !hasScrollbar())
         return;
     if (!m_scrollCorner && hasOverlayScrollbars())
         return;
 
     LayoutObject* actualRenderer = rendererForScrollbar(box());
@@ skipping 325 matching lines @@
 void LayerScrollableArea::setTopmostScrollChild(Layer* scrollChild)
 {
     // We only want to track the topmost scroll child for scrollable areas with
     // overlay scrollbars.
     if (!hasOverlayScrollbars())
         return;
     m_nextTopmostScrollChild = scrollChild;
 }
 
 } // namespace blink