Clip abspos descendants correctly in all columns (not just the first).

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

Index: third_party/WebKit/Source/core/paint/PaintLayerClipper.cpp
diff --git a/third_party/WebKit/Source/core/paint/PaintLayerClipper.cpp b/third_party/WebKit/Source/core/paint/PaintLayerClipper.cpp
index 0ff40ac98a76e1f0088eed1ff66a42638ab3a422..769234c559a067b5e1fdb31b97e52dfe362e5f88 100644
--- a/third_party/WebKit/Source/core/paint/PaintLayerClipper.cpp
+++ b/third_party/WebKit/Source/core/paint/PaintLayerClipper.cpp
@@ -268,10 +268,17 @@ void PaintLayerClipper::calculateClipRects(const ClipRectsContext& context, Clip
     adjustClipRectsForChildren(m_layoutObject, clipRects);
 
     if ((m_layoutObject.hasOverflowClip() && shouldRespectOverflowClip(context)) || m_layoutObject.hasClip()) {
+        const PaintLayer* layer = m_layoutObject.layer();
+        const PaintLayer* layerToStopAt = context.rootLayer;
+        if (const PaintLayer* paginationLayer = layer->enclosingPaginationLayer()) {
+            // Stop at the multicol container, unless the root layer is also inside it.
+            if (layerToStopAt->enclosingPaginationLayer() != paginationLayer)
+                layerToStopAt = paginationLayer;

[chrishtr, 2015/12/14 21:39:51]

Why not just require users of PaintLayerClipper to not ask questions across
pagination boundaries?

+        }
         // This offset cannot use convertToLayerCoords, because sometimes our rootLayer may be across
         // some transformed layer boundary, for example, in the PaintLayerCompositor overlapMap, where
         // clipRects are needed in view space.
-        applyClipRects(context, m_layoutObject, roundedLayoutPoint(m_layoutObject.localToContainerPoint(FloatPoint(), context.rootLayer->layoutObject())), clipRects);
+        applyClipRects(context, m_layoutObject, roundedLayoutPoint(m_layoutObject.localToContainerPoint(FloatPoint(), layerToStopAt->layoutObject())), clipRects);
     }
 }