Improve strut handling in initial column balancing pass.

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

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "core/layout/MultiColumnFragmentainerGroup.h"
 
 #include "core/layout/ColumnBalancer.h"
 #include "core/layout/FragmentationContext.h"
 #include "core/layout/LayoutMultiColumnSet.h"
 
@@ skipping 28 matching lines @@
 
 void MultiColumnFragmentainerGroup::resetColumnHeight() {
   m_maxColumnHeight = calculateMaxColumnHeight();
 
   LayoutMultiColumnFlowThread* flowThread = m_columnSet.multiColumnFlowThread();
   if (m_columnSet.heightIsAuto()) {
     FragmentationContext* enclosingFragmentationContext =
         flowThread->enclosingFragmentationContext();
     if (enclosingFragmentationContext &&
         enclosingFragmentationContext->isFragmentainerLogicalHeightKnown()) {
-      // Even if height is auto, we set an initial height, in order to tell how
-      // much content this MultiColumnFragmentainerGroup can hold, and when we
-      // need to append a new one.
+      // Set an initial height, based on the fragmentainer height in the outer
+      // fragmentation context, in order to tell how much content this
+      // MultiColumnFragmentainerGroup can hold, and when we need to append a
+      // new one.
       m_columnHeight = m_maxColumnHeight;
-    } else {
-      m_columnHeight = LayoutUnit();
+      return;
     }
+  }
+  // If the multicol container has a definite height, use it as the column
+  // height. This even applies when we are to balance the columns. We'll still
+  // use the definite height as an initial height, and lay out once at that
+  // column height. If it turns out that the content needs less than this
+  // height, we have to balance and shrink the height and lay out the columns
+  // over again.
+  if (LayoutUnit logicalHeight = flowThread->columnHeightAvailable()) {
+    setAndConstrainColumnHeight(heightAdjustedForRowOffset(logicalHeight));
   } else {
-    setAndConstrainColumnHeight(
-        heightAdjustedForRowOffset(flowThread->columnHeightAvailable()));
+    m_columnHeight = LayoutUnit();
   }
 }
 
 bool MultiColumnFragmentainerGroup::recalculateColumnHeight(
     LayoutMultiColumnSet& columnSet) {
   LayoutUnit oldColumnHeight = m_columnHeight;
 
   m_maxColumnHeight = calculateMaxColumnHeight();
 
   // Only the last row may have auto height, and thus be balanced. There are no
@@ skipping 517 matching lines @@
 MultiColumnFragmentainerGroupList::addExtraGroup() {
   append(MultiColumnFragmentainerGroup(m_columnSet));
   return last();
 }
 
 void MultiColumnFragmentainerGroupList::deleteExtraGroups() {
   shrink(1);
 }
 
 }  // namespace blink