Invalidate column rules when the width of a multicol container changes.

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

 /*
  * Copyright (C) 2012 Apple Inc.  All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 324 matching lines @@
 
 void LayoutMultiColumnSet::endFlow(LayoutUnit offsetInFlowThread)
 {
     // At this point layout is exactly at the end of this set. Store block offset from flow thread
     // start. This set is now considered "flowed", although we may have to revisit it later (with
     // beginFlow()), e.g. if a subtree in the flow thread has to be laid out over again because the
     // initial margin collapsing estimates were wrong.
     m_fragmentainerGroups.last().setLogicalBottomInFlowThread(offsetInFlowThread);
 }
 
+void LayoutMultiColumnSet::styleDidChange(StyleDifference diff, const ComputedStyle* oldStyle)
+{
+    LayoutBlockFlow::styleDidChange(diff, oldStyle);
+
+    // column-rule is specified on the parent (the multicol container) of this object, but it's the
+    // column sets that are in charge of painting them. A column rule is pretty much like any other
+    // box decoration, like borders. We need to say that we have box decorations here, so that the
+    // columnn set is invalidated when it gets laid out. We cannot check here whether the multicol
+    // container actually has a visible column rule or not, because we may not have been inserted
+    // into the tree yet. Painting a column set is cheap anyway, because the only thing it can
+    // paint is the column rule, while actual multicol content is handled by the flow thread.
+    setHasBoxDecorationBackground(true);
+}
+
 void LayoutMultiColumnSet::layout()
 {
     if (recalculateColumnHeight())
         multiColumnFlowThread()->setColumnHeightsChanged();
     LayoutBlockFlow::layout();
 }
 
 void LayoutMultiColumnSet::computeIntrinsicLogicalWidths(LayoutUnit& minLogicalWidth, LayoutUnit& maxLogicalWidth) const
 {
     minLogicalWidth = m_flowThread->minPreferredLogicalWidth();
@@ skipping 98 matching lines @@
 
 LayoutRect LayoutMultiColumnSet::flowThreadPortionRect() const
 {
     LayoutRect portionRect(LayoutUnit(), logicalTopInFlowThread(), pageLogicalWidth(), logicalHeightInFlowThread());
     if (!isHorizontalWritingMode())
         return portionRect.transposedRect();
     return portionRect;
 }
 
 } // namespace blink