Handle auto-positioned out-of-flow objects inside multicol containers correctly.

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

 /*
  * Copyright (C) 2011 Adobe Systems Incorporated. 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.
@@ skipping 169 matching lines @@
 {
     ASSERT(!RuntimeEnabledFeatures::slimmingPaintV2Enabled() || !m_columnSetsInvalidated);
 
     LayoutRect result;
     for (auto* columnSet : m_multiColumnSetList)
         result.unite(columnSet->fragmentsBoundingBox(layerBoundingBox));
 
     return result;
 }
 
+void LayoutFlowThread::flowThreadToVisualLogicalPosition(LayoutUnit& blockPosition, LayoutUnit& inlinePosition)
+{
+    LayoutPoint position(inlinePosition, blockPosition);
+    if (!isHorizontalWritingMode())
+        position = position.transposedPoint();
+    position = flipForWritingMode(position);

[eae, 2016/08/16 18:23:45]

Instead of:

position = flipForWritingMode(position);
position.move(columnOffset(position));
position = flipForWritingMode(position);

why not do
position.move(columnOffset(flipForWritingMode(position)));
? or would that not have the same effect? The repeated updates of position makes
this code pretty hard to follow.

[mstensho (USE GERRIT), 2016/08/16 18:55:27]

columnOffset() works on physical coordinates, while
flowThreadToVisualLogicalPosition() and its callers are purely logical. So we
have to flip back and forth. I'll add some comments instead. See if that works
better.

+    position.move(columnOffset(position));
+    position = flipForWritingMode(position);
+    if (!isHorizontalWritingMode())
+        position = position.transposedPoint();
+    blockPosition = position.y();
+    inlinePosition = position.x();
+}
+
 void LayoutFlowThread::MultiColumnSetSearchAdapter::collectIfNeeded(const MultiColumnSetInterval& interval)
 {
     if (m_result)
         return;
     if (interval.low() <= m_offset && interval.high() > m_offset)
         m_result = interval.data();
 }
 
 } // namespace blink