Improve multicol overflow rect calculation when column width is 0.

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 249 matching lines @@
         fragment.paginationClip = flowThreadPortionOverflowRectAt(i);
         // Flip it into more a physical (PaintLayer-style) rectangle.
         flowThread->flipForWritingMode(fragment.paginationClip);
 
         fragments.append(fragment);
     }
 }
 
 LayoutRect MultiColumnFragmentainerGroup::calculateOverflow() const
 {
-    unsigned columnCount = actualColumnCount();
-    if (!columnCount)
-        return LayoutRect();
-    return columnRectAt(columnCount - 1);
+    // Note that we just return the bounding rectangle of the column boxes here. We currently don't
+    // examine overflow caused by the actual content that ends up in each column.
+    LayoutRect overflowRect;
+    if (unsigned columnCount = actualColumnCount()) {
+        overflowRect = columnRectAt(0);
+        if (columnCount > 1)
+            overflowRect.uniteEvenIfEmpty(columnRectAt(columnCount - 1));
+    }
+    return overflowRect;
 }
 
 unsigned MultiColumnFragmentainerGroup::actualColumnCount() const
 {
     // We must always return a value of 1 or greater. Column count = 0 is a meaningless situation,
     // and will confuse and cause problems in other parts of the code.
     if (!m_columnHeight)
         return 1;
 
     // Our flow thread portion determines our column count. We have as many columns as needed to fit
@@ skipping 251 matching lines @@
     append(MultiColumnFragmentainerGroup(m_columnSet));
     return last();
 }
 
 void MultiColumnFragmentainerGroupList::deleteExtraGroups()
 {
     shrink(1);
 }
 
 } // namespace blink