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

Source/core/rendering/RenderBlockFlow.cpp

Index: Source/core/rendering/RenderBlockFlow.cpp
diff --git a/Source/core/rendering/RenderBlockFlow.cpp b/Source/core/rendering/RenderBlockFlow.cpp
index 0d371aa66d299744b416c0eaf228cff9323deac4..2219e2261bdefdca37ef9ca87165493effcfaacf 100644
--- a/Source/core/rendering/RenderBlockFlow.cpp
+++ b/Source/core/rendering/RenderBlockFlow.cpp
@@ -1976,6 +1976,15 @@ void RenderBlockFlow::styleDidChange(StyleDifference diff, const RenderStyle* ol
 
     if (diff.needsFullLayout() || !oldStyle)
         createOrDestroyMultiColumnFlowThreadIfNeeded(oldStyle);
+    if (oldStyle) {
+        if (RenderMultiColumnFlowThread* flowThread = multiColumnFlowThread()) {
+            if (!style()->columnRuleEquivalent(oldStyle)) {

[mstensho (USE GERRIT), 2015/01/15 13:31:51]

An alternative solution to this one (additional comparisons in RenderStyle and
the new method in RenderMultiColumnFlowThread) could be to let
RenderMultiColumnSet "inherit" -webkit-column-rule from its parent multicol
(e.g. via RenderObject::propagateStyleToAnonymousChildren()). Then
RenderObject::setStyle() would get called on RenderMultiColumnSet, which in turn
would call setShouldDoFullPaintInvalidation() on it.

Another solution could be to use the displayItemClient() of the multicol
container (i.e. the parent) in MultiColumnSetPainter::paintColumnRules(), but
that seems a bit dirty to me (would need to be able to guarantee that sets get
invalidated and repainted when the multicol container gets invalidated).

I think what I came up with here is cleaner than those two, though.

+                // Column rules are painted by anonymous column set children of the multicol
+                // container. We need to notify them.
+                flowThread->columnRuleStyleDidChange();
+            }
+        }
+    }
 }
 
 void RenderBlockFlow::updateBlockChildDirtyBitsBeforeLayout(bool relayoutChildren, RenderBox& child)