Intersect with the dirty rect before deciding which columns to examine for collection.

Source/core/layout/MultiColumnFragmentainerGroup.h

 // 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.
 
 #ifndef MultiColumnFragmentainerGroup_h
 #define MultiColumnFragmentainerGroup_h
 
 #include "core/layout/LayoutMultiColumnFlowThread.h"
 
 namespace blink {
@@ skipping 81 matching lines @@
         ClampToExistingColumns, // Stay within the range of already existing columns.
         AssumeNewColumns // Allow column indices outside the range of already existing columns.
     };
     unsigned columnIndexAtOffset(LayoutUnit offsetInFlowThread, ColumnIndexCalculationMode = ClampToExistingColumns) const;
 
     // Return the column that the specified visual point belongs to. Only the coordinate on the
     // column progression axis is relevant. Every point belongs to a column, even if said point is
     // not inside any of the columns.
     unsigned columnIndexAtVisualPoint(const LayoutPoint& visualPoint) const;
 
+    // Get the first and the last column intersecting the specified visual rectangle.
+    void columnIntervalForVisualRect(const LayoutRect&, unsigned& firstColumn, unsigned& lastColumn) const;
+
     LayoutMultiColumnSet& m_columnSet;
 
     LayoutUnit m_logicalTop;
     LayoutUnit m_logicalTopInFlowThread;
     LayoutUnit m_logicalBottomInFlowThread;
 
     LayoutUnit m_columnHeight;
 
     // The following variables are used when balancing the column set.
     LayoutUnit m_maxColumnHeight; // Maximum column height allowed.
@@ skipping 61 matching lines @@
 
 private:
     LayoutMultiColumnSet& m_columnSet;
 
     Vector<MultiColumnFragmentainerGroup, 1> m_groups;
 };
 
 } // namespace blink
 
 #endif // MultiColumnFragmentainerGroup_h