Avoid stretching the parent of a column spanner.

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 "config.h"
 
 #include "core/layout/MultiColumnFragmentainerGroup.h"
 
 #include "core/layout/LayoutMultiColumnSet.h"
 
@@ skipping 444 matching lines @@
     return m_columnSet.isHorizontalWritingMode() ? offset : offset.transposedSize();
 }
 
 LayoutRect MultiColumnFragmentainerGroup::columnRectAt(unsigned columnIndex) const
 {
     LayoutUnit columnLogicalWidth = m_columnSet.pageLogicalWidth();
     LayoutUnit columnLogicalHeight = m_columnHeight;
     LayoutUnit columnLogicalTop;
     LayoutUnit columnLogicalLeft;
     LayoutUnit columnGap = m_columnSet.columnGap();
+    LayoutUnit portionOutsideFlowThread = logicalTopInFlowThread() + (columnIndex + 1) * columnLogicalHeight - logicalBottomInFlowThread();
+    if (portionOutsideFlowThread > 0) {
+        // The last column may not be using all available space.
+        ASSERT(columnIndex + 1 == actualColumnCount());
+        columnLogicalHeight -= portionOutsideFlowThread;
+        ASSERT(columnLogicalHeight >= 0);
+    }
 
     if (m_columnSet.multiColumnFlowThread()->progressionIsInline()) {
         if (m_columnSet.style()->isLeftToRightDirection())
             columnLogicalLeft += columnIndex * (columnLogicalWidth + columnGap);
         else
             columnLogicalLeft += m_columnSet.contentLogicalWidth() - columnLogicalWidth - columnIndex * (columnLogicalWidth + columnGap);
     } else {
-        columnLogicalTop += columnIndex * (columnLogicalHeight + columnGap);
+        columnLogicalTop += columnIndex * (m_columnHeight + columnGap);
     }
 
     LayoutRect columnRect(columnLogicalLeft, columnLogicalTop, columnLogicalWidth, columnLogicalHeight);
     if (!m_columnSet.isHorizontalWritingMode())
         return columnRect.transposedRect();
     return columnRect;
 }
 
 LayoutRect MultiColumnFragmentainerGroup::flowThreadPortionRectAt(unsigned columnIndex) const
 {
     LayoutUnit logicalTop = logicalTopInFlowThreadAt(columnIndex);
+    LayoutUnit logicalBottom = logicalTop + m_columnHeight;
+    if (logicalBottom > logicalBottomInFlowThread()) {
+        // The last column may not be using all available space.
+        ASSERT(columnIndex + 1 == actualColumnCount());
+        logicalBottom = logicalBottomInFlowThread();
+        ASSERT(logicalBottom >= logicalTop);
+    }
+    LayoutUnit portionLogicalHeight = logicalBottom - logicalTop;
     if (m_columnSet.isHorizontalWritingMode())
-        return LayoutRect(LayoutUnit(), logicalTop, m_columnSet.pageLogicalWidth(), m_columnHeight);
-    return LayoutRect(logicalTop, LayoutUnit(), m_columnHeight, m_columnSet.pageLogicalWidth());
+        return LayoutRect(LayoutUnit(), logicalTop, m_columnSet.pageLogicalWidth(), portionLogicalHeight);
+    return LayoutRect(logicalTop, LayoutUnit(), portionLogicalHeight, m_columnSet.pageLogicalWidth());
 }
 
 LayoutRect MultiColumnFragmentainerGroup::flowThreadPortionOverflowRect(const LayoutRect& portionRect, unsigned columnIndex, unsigned columnCount, LayoutUnit columnGap) const
 {
     // This function determines the portion of the flow thread that paints for the column. Along the inline axis, columns are
     // unclipped at outside edges (i.e., the first and last column in the set), and they clip to half the column
     // gap along interior edges.
     //
     // In the block direction, we will not clip overflow out of the top of the first column, or out of the bottom of
     // the last column. This applies only to the true first column and last column across all column sets.
@@ skipping 81 matching lines @@
     append(MultiColumnFragmentainerGroup(m_columnSet));
     return last();
 }
 
 void MultiColumnFragmentainerGroupList::deleteExtraGroups()
 {
     shrink(1);
 }
 
 } // namespace blink