[New Multicolumn] Correct paint rectangle calculation for compositing.

Source/core/rendering/RenderMultiColumnSet.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
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         // Now get the overflow rect that corresponds to the column.
         LayoutRect flowThreadOverflowPortion = flowThreadPortionOverflowRect(flowThreadPortion, i, colCount, colGap);
 
         // Do a paint invalidation for this specific column.
         paintInvalidationOfFlowThreadContentRectangle(paintInvalidationRect, flowThreadPortion, flowThreadOverflowPortion, colRect.location());
     }
 }
 
 void RenderMultiColumnSet::collectLayerFragments(LayerFragments& fragments, const LayoutRect& layerBoundingBox, const LayoutRect& dirtyRect)
 {
-    // The two rectangles passed to this method are physical, except that we pretend that there's
-    // only one long column (that's how a flow thread works).
+    // |layerBoundingBox| is in the flow thread coordinate space, relative to the top/left edge of
+    // the flow thread, but note that it has been converted with respect to writing mode (so that
+    // it's visual/physical in that sense).
+    //
+    // |dirtyRect| is visual, relative to the multicol container.
     //
     // Then there's the output from this method - the stuff we put into the list of fragments. The
-    // fragment.paginationOffset point is the actual physical translation required to get from a
+    // fragment.paginationOffset point is the actual visual translation required to get from a
     // location in the flow thread to a location in a given column. The fragment.paginationClip
-    // rectangle, on the other hand, is in the same coordinate system as the two rectangles passed
-    // to this method (flow thread coordinates).
+    // rectangle, on the other hand, is in flow thread coordinates.
     //
     // All other rectangles in this method are sized physically, and the inline direction coordinate
     // is physical too, but the block direction coordinate is "logical top". This is the same as
     // e.g. RenderBox::frameRect(). These rectangles also pretend that there's only one long column,
     // i.e. they are for the flow thread.
 
     // Put the layer bounds into flow thread-local coordinates by flipping it first. Since we're in
     // a renderer, most rectangles are represented this way.
     LayoutRect layerBoundsInFlowThread(layerBoundingBox);
     flowThread()->flipForWritingMode(layerBoundsInFlowThread);
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 void RenderMultiColumnSet::detachRegion()
 {
     if (m_flowThread) {
         m_flowThread->removeRegionFromThread(this);
         m_flowThread = 0;
     }
 }
 
 }