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

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

 /*
  * Copyright (C) 2006, 2007, 2008, 2009, 2010, 2011, 2012 Apple Inc. All rights reserved.
  *
  * Portions are Copyright (C) 1998 Netscape Communications Corporation.
  *
  * Other contributors:
  *   Robert O'Callahan <roc+@cs.cmu.edu>
  *   David Baron <dbaron@fas.harvard.edu>
  *   Christian Biesinger <cbiesinger@web.de>
  *   Randall Jesup <rjesup@wgate.com>
@@ skipping 250 matching lines @@
         } else {
             parentLayer->clipper().calculateClipRects(context, clipRects);
         }
     } else {
         clipRects.reset(LayoutRect(LayoutRect::infiniteIntRect()));
     }
 
     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);
     }
 }
 
 static ClipRect backgroundClipRectForPosition(const ClipRects& parentRects, EPosition position)
 {
     if (position == FixedPosition)
         return parentRects.fixedClipRect();
 
     if (position == AbsolutePosition)
         return parentRects.posClipRect();
@@ skipping 46 matching lines @@
 
 ClipRects* PaintLayerClipper::paintingClipRects(const PaintLayer* rootLayer, ShouldRespectOverflowClip respectOverflowClip, const LayoutSize& subpixelAccumulation) const
 {
     ClipRectsContext context(rootLayer, PaintingClipRects, IgnoreOverlayScrollbarSize, subpixelAccumulation);
     if (respectOverflowClip == IgnoreOverflowClip)
         context.setIgnoreOverflowClip();
     return getClipRects(context);
 }
 
 } // namespace blink