Avoid over-eager clipping of child layers in multicol.

third_party/WebKit/Source/core/layout/LayoutMultiColumnSet.cpp

 /*
  * Copyright (C) 2012 Apple Inc.  All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 294 matching lines @@
 LayoutRect LayoutMultiColumnSet::flowThreadPortionOverflowRect() const {
   return overflowRectForFlowThreadPortion(flowThreadPortionRect(),
                                           !previousSiblingMultiColumnSet(),
                                           !nextSiblingMultiColumnSet());
 }
 
 LayoutRect LayoutMultiColumnSet::overflowRectForFlowThreadPortion(
     const LayoutRect& flowThreadPortionRect,
     bool isFirstPortion,
     bool isLastPortion) const {
-  if (hasOverflowClip())
-    return flowThreadPortionRect;
+  // Only clip along the block direction axis.
+  LayoutRect clipRect(LayoutRect::infiniteIntRect());
 
-  LayoutRect flowThreadOverflow = m_flowThread->visualOverflowRect();
-
-  // Only clip along the flow thread axis.
-  LayoutRect clipRect;
   if (m_flowThread->isHorizontalWritingMode()) {
-    LayoutUnit minY =
-        isFirstPortion ? flowThreadOverflow.y() : flowThreadPortionRect.y();
-    LayoutUnit maxY = isLastPortion ? std::max(flowThreadPortionRect.maxY(),
-                                               flowThreadOverflow.maxY())
-                                    : flowThreadPortionRect.maxY();
-    LayoutUnit minX =
-        std::min(flowThreadPortionRect.x(), flowThreadOverflow.x());
-    LayoutUnit maxX =
-        std::max(flowThreadPortionRect.maxX(), flowThreadOverflow.maxX());
-    clipRect = LayoutRect(minX, minY, maxX - minX, maxY - minY);
-  } else {
-    LayoutUnit minX =
-        isFirstPortion ? flowThreadOverflow.x() : flowThreadPortionRect.x();
-    LayoutUnit maxX = isLastPortion ? std::max(flowThreadPortionRect.maxX(),
-                                               flowThreadOverflow.maxX())
-                                    : flowThreadPortionRect.maxX();
-    LayoutUnit minY =
-        std::min(flowThreadPortionRect.y(), (flowThreadOverflow.y()));
-    LayoutUnit maxY =
-        std::max(flowThreadPortionRect.y(), (flowThreadOverflow.maxY()));
-    clipRect = LayoutRect(minX, minY, maxX - minX, maxY - minY);
+    if (!isFirstPortion)
+      clipRect.shiftYEdgeTo(flowThreadPortionRect.y());
+    if (!isLastPortion)
+      clipRect.shiftMaxYEdgeTo(flowThreadPortionRect.maxY());
+    return clipRect;
   }
-
+  if (!isFirstPortion)
+    clipRect.shiftXEdgeTo(flowThreadPortionRect.x());
+  if (!isLastPortion)
+    clipRect.shiftMaxXEdgeTo(flowThreadPortionRect.maxX());
   return clipRect;
 }
 
 bool LayoutMultiColumnSet::heightIsAuto() const {
   LayoutMultiColumnFlowThread* flowThread = multiColumnFlowThread();
   if (!flowThread->isLayoutPagedFlowThread()) {
     // If support for the column-fill property isn't enabled, we want to behave
     // as if column-fill were auto, so that multicol containers with specified
     // height don't get their columns balanced (auto-height multicol containers
     // will still get their columns balanced, even if column-fill isn't
@@ skipping 271 matching lines @@
   // Now add in column rule bounds, if present.
   Vector<LayoutRect> columnRuleBounds;
   if (computeColumnRuleBounds(LayoutPoint(), columnRuleBounds)) {
     for (auto& bound : columnRuleBounds)
       blockFlowBounds.unite(bound);
   }
   return blockFlowBounds;
 }
 
 }  // namespace blink