[Slimming paint] [New Multicolumn] Repaint column rules when they change.

Source/core/paint/MultiColumnSetPainter.cpp

 // Copyright 2014 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/paint/MultiColumnSetPainter.h"
 
 #include "core/paint/BlockPainter.h"
 #include "core/paint/BoxPainter.h"
 #include "core/paint/RenderDrawingRecorder.h"
@@ skipping 34 matching lines @@
     LayoutUnit colGap = m_renderMultiColumnSet.columnGap();
     bool renderRule = ruleStyle > BHIDDEN && !ruleTransparent;
     if (!renderRule)
         return;
 
     unsigned colCount = m_renderMultiColumnSet.actualColumnCount();
     if (colCount <= 1)
         return;
 
     DrawingRecorder drawingRecorder(paintInfo.context, m_renderMultiColumnSet.displayItemClient(), DisplayItem::ColumnRules, m_renderMultiColumnSet.visualOverflowRect());
+    if (drawingRecorder.canUseCachedDrawing())
+        return;
+
     bool antialias = BoxPainter::shouldAntialiasLines(paintInfo.context);
     bool leftToRight = m_renderMultiColumnSet.style()->isLeftToRightDirection();
     LayoutUnit currLogicalLeftOffset = leftToRight ? LayoutUnit() : m_renderMultiColumnSet.contentLogicalWidth();
     LayoutUnit ruleAdd = m_renderMultiColumnSet.borderAndPaddingLogicalLeft();
     LayoutUnit ruleLogicalLeft = leftToRight ? LayoutUnit() : m_renderMultiColumnSet.contentLogicalWidth();
     LayoutUnit inlineDirectionSize = m_renderMultiColumnSet.pageLogicalWidth();
     BoxSide boxSide = m_renderMultiColumnSet.isHorizontalWritingMode()
         ? leftToRight ? BSLeft : BSRight
         : leftToRight ? BSTop : BSBottom;
 
@@ skipping 15 matching lines @@
             LayoutUnit ruleBottom = m_renderMultiColumnSet.isHorizontalWritingMode() ? ruleTop + m_renderMultiColumnSet.contentHeight() : ruleTop + ruleThickness;
             IntRect pixelSnappedRuleRect = pixelSnappedIntRectFromEdges(ruleLeft, ruleTop, ruleRight, ruleBottom);
             ObjectPainter::drawLineForBoxSide(paintInfo.context, pixelSnappedRuleRect.x(), pixelSnappedRuleRect.y(), pixelSnappedRuleRect.maxX(), pixelSnappedRuleRect.maxY(), boxSide, ruleColor, ruleStyle, 0, 0, antialias);
         }
 
         ruleLogicalLeft = currLogicalLeftOffset;
     }
 }
 
 } // namespace blink