Adjust column rows' height better for their offset in the multicol container.

third_party/WebKit/Source/core/layout/LayoutMultiColumnSet.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 152 matching lines @@
         LayoutUnit blockOffsetInFlowThread = previousGroup.logicalTopInFlowThread() + previousGroup.logicalHeight() * usedColumnCount();
         previousGroup.setLogicalBottomInFlowThread(blockOffsetInFlowThread);
         newGroup.setLogicalTopInFlowThread(blockOffsetInFlowThread);
         newGroup.setLogicalTop(previousGroup.logicalTop() + previousGroup.logicalHeight());
         newGroup.resetColumnHeight();
     }
     m_fragmentainerGroups.append(newGroup);
     return m_fragmentainerGroups.last();
 }
 
+LayoutUnit LayoutMultiColumnSet::logicalTopFromMulticolContentEdge() const
+{
+    // We subtract the position of the first column set or spanner placeholder, rather than the
+    // "before" border+padding of the multicol container. This distinction doesn't matter after
+    // layout, but during layout it does: The flow thread (i.e. the multicol contents) is laid out
+    // before the column sets and spanner placeholders, which means that compesating for a top
+    // border+padding that hasn't yet been baked into the offset will produce the wrong results in
+    // the first layout pass, and we'd end up performing a wasted layout pass in many cases.
+    const LayoutBox& firstColumnBox = *multiColumnFlowThread()->firstMultiColumnBox();
+    // The top margin edge of the first column set or spanner placeholder is flush with the top
+    // content edge of the multicol container. The margin here never collapses with other margins,
+    // so we can just subtract it. Column sets never have margins, but spanner placeholders may.
+    LayoutUnit firstColumnBoxMarginEdge = firstColumnBox.logicalTop() - multiColumnBlockFlow()->marginBeforeForChild(firstColumnBox);
+    return logicalTop() - firstColumnBoxMarginEdge;
+}
+
 LayoutUnit LayoutMultiColumnSet::logicalTopInFlowThread() const
 {
     return firstFragmentainerGroup().logicalTopInFlowThread();
 }
 
 LayoutUnit LayoutMultiColumnSet::logicalBottomInFlowThread() const
 {
     return lastFragmentainerGroup().logicalBottomInFlowThread();
 }
 
@@ skipping 220 matching lines @@
 
 LayoutRect LayoutMultiColumnSet::flowThreadPortionRect() const
 {
     LayoutRect portionRect(LayoutUnit(), logicalTopInFlowThread(), pageLogicalWidth(), logicalHeightInFlowThread());
     if (!isHorizontalWritingMode())
         return portionRect.transposedRect();
     return portionRect;
 }
 
 }