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

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

Index: third_party/WebKit/Source/core/paint/BlockPainter.cpp
diff --git a/third_party/WebKit/Source/core/paint/BlockPainter.cpp b/third_party/WebKit/Source/core/paint/BlockPainter.cpp
index 75855f4a5374bf2bfca7f90cfc915527436a0470..016cefe0e6cafa8744f22cdb834a1b666780abab 100644
--- a/third_party/WebKit/Source/core/paint/BlockPainter.cpp
+++ b/third_party/WebKit/Source/core/paint/BlockPainter.cpp
@@ -74,6 +74,7 @@ void BlockPainter::paintOverflowControlsIfNeeded(const PaintInfo& paintInfo, con
 {
     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 */);
     }
 }