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

third_party/WebKit/Source/core/layout/LayoutMultiColumnFlowThread.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 278 matching lines @@
     return newWidth != logicalWidth();
 }
 
 bool LayoutMultiColumnFlowThread::isPageLogicalHeightKnown() const
 {
     if (LayoutMultiColumnSet* columnSet = lastMultiColumnSet())
         return columnSet->isPageLogicalHeightKnown();
     return false;
 }
 
+LayoutSize LayoutMultiColumnFlowThread::offsetFromContainer(const LayoutObject* o, const LayoutPoint& point, bool* offsetDependsOnPoint) const
+{
+    ASSERT(o == container());
+
+    // So far the point has been in flow thread coordinates (i.e. as if everything in
+    // the fragmentation context lived in one tall single column). Convert it to a
+    // visual point now.
+
+    // TODO(mstensho): It's bad to assume that we want offsetFromContainer() to convert to visual
+    // coordinates. See LayoutBoxModelObject::mapAbsoluteToLocalPoint(). We really only want such
+    // conversions when walking *upwards* in the tree, but offsetFromContainer() has no idea why
+    // it's being called.
+    if (offsetDependsOnPoint)
+        *offsetDependsOnPoint = true;
+    return columnOffset(point) + topLeftLocationOffset();
+}
+
 LayoutSize LayoutMultiColumnFlowThread::flowThreadTranslationAtOffset(LayoutUnit offsetInFlowThread) const
 {
     LayoutMultiColumnSet* columnSet = columnSetAtBlockOffset(offsetInFlowThread);
     if (!columnSet)
         return LayoutSize(0, 0);
     return columnSet->flowThreadTranslationAtOffset(offsetInFlowThread);
 }
 
 LayoutPoint LayoutMultiColumnFlowThread::visualPointToFlowThreadPoint(const LayoutPoint& visualPoint) const
 {
@@ skipping 636 matching lines @@
     // the "mayBe"), if we're in an out-of-flow subtree and have an outer multicol container that
     // doesn't affect us, but that's okay. We'll discover that further down the road when trying to
     // locate our enclosing flow thread for real.
     bool mayBeNested = multiColumnBlockFlow()->isInsideFlowThread() || view()->fragmentationContext();
     if (!mayBeNested)
         return;
     appendNewFragmentainerGroupIfNeeded(logicalTopInFlowThreadAfterPagination);
 }
 
 }