Allow flow thread portion logical bottom to be above its logical top.

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 477 matching lines @@
       columnSet = previousSet;
   }
   // Avoid returning zero-height column sets, if possible. We found a column set
   // based on a flow thread coordinate. If multiple column sets share that
   // coordinate (because we have zero-height column sets between column
   // spanners, for instance), look for one that has a height.
   for (LayoutMultiColumnSet* walker = columnSet; walker;
        walker = walker->nextSiblingMultiColumnSet()) {
     if (!walker->isPageLogicalHeightKnown())
       continue;
-    if (walker->logicalTopInFlowThread() == offset)
+    if (pageBoundaryRule == AssociateWithFormerPage) {
+      if (walker->logicalTopInFlowThread() < offset &&
+          walker->logicalBottomInFlowThread() >= offset)
+        return walker;
+    }
+    if (walker->logicalTopInFlowThread() <= offset &&
+        walker->logicalBottomInFlowThread() > offset)
       return walker;
     break;
   }
   return columnSet;
 }
 
 void LayoutMultiColumnFlowThread::layoutColumns(
     SubtreeLayoutScope& layoutScope) {
   // Since we ended up here, it means that the multicol container (our parent)
   // needed layout. Since contents of the multicol container are diverted to the
@@ skipping 380 matching lines @@
   LayoutFlowThread::willBeRemovedFromTree();
 }
 
 void LayoutMultiColumnFlowThread::skipColumnSpanner(
     LayoutBox* layoutObject,
     LayoutUnit logicalTopInFlowThread) {
   ASSERT(layoutObject->isColumnSpanAll());
   LayoutMultiColumnSpannerPlaceholder* placeholder =
       layoutObject->spannerPlaceholder();
   LayoutBox* previousColumnBox = placeholder->previousSiblingMultiColumnBox();
-  if (previousColumnBox && previousColumnBox->isLayoutMultiColumnSet()) {
-    LayoutMultiColumnSet* columnSet = toLayoutMultiColumnSet(previousColumnBox);
-    // Negative margins may cause this.
-    if (logicalTopInFlowThread < columnSet->logicalTopInFlowThread())
-      logicalTopInFlowThread = columnSet->logicalTopInFlowThread();
-    columnSet->endFlow(logicalTopInFlowThread);
-  }
+  if (previousColumnBox && previousColumnBox->isLayoutMultiColumnSet())
+    toLayoutMultiColumnSet(previousColumnBox)->endFlow(logicalTopInFlowThread);
   LayoutBox* nextColumnBox = placeholder->nextSiblingMultiColumnBox();
   if (nextColumnBox && nextColumnBox->isLayoutMultiColumnSet()) {
     LayoutMultiColumnSet* nextSet = toLayoutMultiColumnSet(nextColumnBox);
     m_lastSetWorkedOn = nextSet;
     nextSet->beginFlow(logicalTopInFlowThread);
   }
 
   // We'll lay out of spanners after flow thread layout has finished (during
   // layout of the spanner placeholders). There may be containing blocks for
   // out-of-flow positioned descendants of the spanner in the flow thread, so
@@ skipping 453 matching lines @@
     const {
   return MultiColumnLayoutState(m_lastSetWorkedOn);
 }
 
 void LayoutMultiColumnFlowThread::restoreMultiColumnLayoutState(
     const MultiColumnLayoutState& state) {
   m_lastSetWorkedOn = state.columnSet();
 }
 
 }  // namespace blink