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

third_party/WebKit/Source/core/paint/BoxClipper.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/BoxClipper.h"
 
 #include "core/layout/LayoutBox.h"
 #include "core/paint/PaintInfo.h"
 #include "core/paint/PaintLayer.h"
 #include "platform/RuntimeEnabledFeatures.h"
 #include "platform/graphics/GraphicsLayer.h"
 #include "platform/graphics/paint/ClipDisplayItem.h"
 #include "platform/graphics/paint/PaintController.h"
 
 namespace blink {
 
 BoxClipper::BoxClipper(const LayoutBox& box, const PaintInfo& paintInfo, const LayoutPoint& accumulatedOffset, ContentsClipBehavior contentsClipBehavior)
     : m_box(box)
     , m_paintInfo(paintInfo)
     , m_clipType(DisplayItem::UninitializedType)
 {
-    if (m_paintInfo.phase == PaintPhaseBlockBackground || m_paintInfo.phase == PaintPhaseSelfOutline || m_paintInfo.phase == PaintPhaseMask)
+    if ((m_paintInfo.phase == PaintPhaseBlockBackground && contentsClipBehavior != ClipForScrollbars)
+        || m_paintInfo.phase == PaintPhaseSelfOutline || m_paintInfo.phase == PaintPhaseMask)
         return;
 
-    bool isControlClip = m_box.hasControlClip();
-    bool isOverflowClip = m_box.hasOverflowClip() && !m_box.layer()->isSelfPaintingLayer();
+    bool hasControlClip = m_box.hasControlClip();
+    bool hasOverflowClip = m_box.hasOverflowClip() && !m_box.layer()->isSelfPaintingLayer();
 
-    if (!isControlClip && !isOverflowClip)
+    if (!hasControlClip && !hasOverflowClip)
         return;
 
-    LayoutRect clipRect = isControlClip ? m_box.controlClipRect(accumulatedOffset) : m_box.overflowClipRect(accumulatedOffset);
+    LayoutRect clipRect;
+
+    if (hasControlClip) {
+        clipRect = m_box.controlClipRect(accumulatedOffset);
+    } else {
+        ASSERT(hasOverflowClip);
+        if (contentsClipBehavior == ClipForScrollbars) {
+            clipRect = m_box.borderBoxRect();
+            clipRect.moveBy(accumulatedOffset);
+        } else {
+            clipRect = m_box.overflowClipRect(accumulatedOffset);
+        }
+    }
+
     FloatRoundedRect clipRoundedRect(0, 0, 0, 0);
-    bool hasBorderRadius = m_box.style()->hasBorderRadius();
-    if (hasBorderRadius)
+    // Scrollbars paint on top of a rounded border.
+    bool shouldClipBorderRadius = m_box.style()->hasBorderRadius() && contentsClipBehavior != ClipForScrollbars;
+    if (shouldClipBorderRadius)
         clipRoundedRect = m_box.style()->getRoundedInnerBorderFor(LayoutRect(accumulatedOffset, m_box.size()));
 
     if (contentsClipBehavior == SkipContentsClipIfPossible) {
         LayoutRect contentsVisualOverflow = m_box.contentsVisualOverflowRect();
         if (contentsVisualOverflow.isEmpty())
             return;
 
         LayoutRect conservativeClipRect = clipRect;
-        if (hasBorderRadius)
+        if (shouldClipBorderRadius)
             conservativeClipRect.intersect(LayoutRect(clipRoundedRect.radiusCenterRect()));
         conservativeClipRect.moveBy(-accumulatedOffset);
         if (m_box.hasLayer())
             conservativeClipRect.move(m_box.scrolledContentOffset());
         if (conservativeClipRect.contains(contentsVisualOverflow))
             return;
     }
 
     if (!m_paintInfo.context->paintController().displayItemConstructionIsDisabled()) {
         m_clipType = m_paintInfo.displayItemTypeForClipping();
         Vector<FloatRoundedRect> roundedRects;
-        if (hasBorderRadius)
+        if (shouldClipBorderRadius)
             roundedRects.append(clipRoundedRect);
         m_paintInfo.context->paintController().createAndAppend<ClipDisplayItem>(m_box, m_clipType, pixelSnappedIntRect(clipRect), roundedRects);
     }
 }
 
 BoxClipper::~BoxClipper()
 {
     if (m_clipType == DisplayItem::UninitializedType)
         return;
 
     ASSERT(m_box.hasControlClip() || (m_box.hasOverflowClip() && !m_box.layer()->isSelfPaintingLayer()));
     m_paintInfo.context->paintController().endItem<EndClipDisplayItem>(m_box, DisplayItem::clipTypeToEndClipType(m_clipType));
 }
 
 } // namespace blink