Clip scrollbars to box bounds when they don't fit.

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

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "config.h"
 #include "core/paint/BlockPainter.h"
 
 #include "core/editing/DragCaretController.h"
 #include "core/editing/FrameSelection.h"
 #include "core/frame/Settings.h"
@@ skipping 56 matching lines @@
     // Our scrollbar widgets paint exactly when we tell them to, so that they work properly with
     // z-index. We paint after we painted the background/border, so that the scrollbars will
     // sit above the background/border.
     paintOverflowControlsIfNeeded(localPaintInfo, adjustedPaintOffset);
 }
 
 void BlockPainter::paintOverflowControlsIfNeeded(const PaintInfo& paintInfo, const LayoutPoint& paintOffset)
 {
     PaintPhase phase = paintInfo.phase;
     if (m_layoutBlock.hasOverflowClip() && m_layoutBlock.style()->visibility() == VISIBLE && (phase == PaintPhaseBlockBackground || phase == PaintPhaseChildBlockBackground) && paintInfo.shouldPaintWithinRoot(&m_layoutBlock) && !paintInfo.paintRootBackgroundOnly()) {
+        BoxClipper boxClipper(m_layoutBlock, paintInfo, paintOffset, ClipForScrollbars);

[chrishtr, 2015/11/19 17:29:52]

Your use of BoxClipper is not quite right. For example, it does not emit a
unique display item id.

In your use case, if line 77 here has been reached, then we should always clip
right? If so, just using a ClipRecorder with a new display item id and
borderBoxRect is a lot simpler.

[skobes, 2015/11/19 18:30:07]

Done.

         ScrollableAreaPainter(*m_layoutBlock.layer()->scrollableArea()).paintOverflowControls(paintInfo.context, roundedIntPoint(paintOffset), paintInfo.cullRect(), false /* paintingOverlayControls */);
     }
 }
 
 void BlockPainter::paintChildren(const PaintInfo& paintInfo, const LayoutPoint& paintOffset)
 {
     for (LayoutBox* child = m_layoutBlock.firstChildBox(); child; child = child->nextSiblingBox())
         paintChild(*child, paintInfo, paintOffset);
 }
 
@@ skipping 174 matching lines @@
 
         // We don't paint our own background, but we do let the kids paint their backgrounds.
         PaintInfo paintInfoForChild(paintInfo);
         paintInfoForChild.phase = newPhase;
         paintInfoForChild.updatePaintingRootForChildren(&m_layoutBlock);
         m_layoutBlock.paintChildren(paintInfoForChild, paintOffset);
     }
 }
 
 } // namespace blink