Remove clips where we can show that they are unnecessary.

Source/core/rendering/RenderBox.cpp

Index: Source/core/rendering/RenderBox.cpp
diff --git a/Source/core/rendering/RenderBox.cpp b/Source/core/rendering/RenderBox.cpp
index 30f81ffee71d501e462244399530fa09f8f2c6bd..311729dc019d3ba0e9f3961e1a9d9e21bdede199 100644
--- a/Source/core/rendering/RenderBox.cpp
+++ b/Source/core/rendering/RenderBox.cpp
@@ -1517,7 +1517,7 @@ bool RenderBox::repaintLayerRectsForImage(WrappedImagePtr image, const FillLayer
     return false;
 }
 
-bool RenderBox::pushContentsClip(PaintInfo& paintInfo, const LayoutPoint& accumulatedOffset)
+bool RenderBox::pushContentsClip(PaintInfo& paintInfo, const LayoutPoint& accumulatedOffset, ContentsClipBehavior contentsClipBehavior)
 {
     if (paintInfo.phase == PaintPhaseBlockBackground || paintInfo.phase == PaintPhaseSelfOutline || paintInfo.phase == PaintPhaseMask)
         return false;
@@ -1528,6 +1528,27 @@ bool RenderBox::pushContentsClip(PaintInfo& paintInfo, const LayoutPoint& accumu
     if (!isControlClip && !isOverflowClip)
         return false;
 
+    LayoutRect clipRect = isControlClip ? controlClipRect(accumulatedOffset) : overflowClipRect(accumulatedOffset, paintInfo.renderRegion);
+    RoundedRect clipRoundedRect(0, 0, 0, 0);
+    bool hasBorderRadius = style()->hasBorderRadius();
+    if (hasBorderRadius)
+        clipRoundedRect = style()->getRoundedInnerBorderFor(LayoutRect(accumulatedOffset, size()));
+
+    if (contentsClipBehavior == SkipContentsClipIfPossible) {
+        LayoutRect contentsVisualOverflow = contentsVisualOverflowRect();
+        if (contentsVisualOverflow.isEmpty())
+            return false;
+
+        LayoutRect conservativeClipRect = clipRect;
+        if (hasBorderRadius)
+            conservativeClipRect.intersect(clipRoundedRect.radiusCenterRect());
+        conservativeClipRect.moveBy(-accumulatedOffset);
+        if (hasLayer())
+            conservativeClipRect.move(scrolledContentOffset());
+        if (conservativeClipRect.contains(contentsVisualOverflow))
+            return false;
+    }
+
     if (paintInfo.phase == PaintPhaseOutline)
         paintInfo.phase = PaintPhaseChildOutlines;
     else if (paintInfo.phase == PaintPhaseChildBlockBackground) {
@@ -1535,11 +1556,10 @@ bool RenderBox::pushContentsClip(PaintInfo& paintInfo, const LayoutPoint& accumu
         paintObject(paintInfo, accumulatedOffset);
         paintInfo.phase = PaintPhaseChildBlockBackgrounds;
     }
-    IntRect clipRect = pixelSnappedIntRect(isControlClip ? controlClipRect(accumulatedOffset) : overflowClipRect(accumulatedOffset, paintInfo.renderRegion));
     paintInfo.context->save();
-    if (style()->hasBorderRadius())
-        paintInfo.context->clipRoundedRect(style()->getRoundedInnerBorderFor(LayoutRect(accumulatedOffset, size())));
-    paintInfo.context->clip(clipRect);
+    if (hasBorderRadius)
+        paintInfo.context->clipRoundedRect(clipRoundedRect);
+    paintInfo.context->clip(pixelSnappedIntRect(clipRect));
     return true;
 }
 
@@ -4210,7 +4230,8 @@ void RenderBox::addVisualEffectOverflow()
     }
 
     // Add in the final overflow with shadows and outsets combined.
-    addVisualOverflow(LayoutRect(overflowMinX, overflowMinY, overflowMaxX - overflowMinX, overflowMaxY - overflowMinY));
+    LayoutRect visualEffectOverflow(overflowMinX, overflowMinY, overflowMaxX - overflowMinX, overflowMaxY - overflowMinY);
+    addVisualOverflow(visualEffectOverflow);
 }
 
 void RenderBox::addOverflowFromChild(RenderBox* child, const LayoutSize& delta)
@@ -4229,11 +4250,11 @@ void RenderBox::addOverflowFromChild(RenderBox* child, const LayoutSize& delta)
     // Add in visual overflow from the child.  Even if the child clips its overflow, it may still
     // have visual overflow of its own set from box shadows or reflections.  It is unnecessary to propagate this
     // overflow if we are clipping our own overflow.
-    if (child->hasSelfPaintingLayer() || hasOverflowClip())
+    if (child->hasSelfPaintingLayer())
         return;
     LayoutRect childVisualOverflowRect = child->visualOverflowRectForPropagation(style());
     childVisualOverflowRect.move(delta);
-    addVisualOverflow(childVisualOverflowRect);
+    addContentsVisualOverflow(childVisualOverflowRect);
 }
 
 void RenderBox::addLayoutOverflow(const LayoutRect& rect)
@@ -4298,12 +4319,24 @@ void RenderBox::addVisualOverflow(const LayoutRect& rect)
     m_overflow->addVisualOverflow(rect);
 }
 
+void RenderBox::addContentsVisualOverflow(const LayoutRect& rect)
+{
+    if (!hasOverflowClip()) {
+        addVisualOverflow(rect);
+        return;
+    }
+
+    if (!m_overflow)
+        m_overflow = adoptPtr(new RenderOverflow(clientBoxRect(), borderBoxRect()));
+    m_overflow->addContentsVisualOverflow(rect);
+}
+
 void RenderBox::clearLayoutOverflow()
 {
     if (!m_overflow)
         return;
 
-    if (!hasVisualOverflow()) {
+    if (!hasVisualOverflow() && contentsVisualOverflowRect().isEmpty()) {
         m_overflow.clear();
         return;
     }