[New Multicolumn] Correct paint rectangle calculation for compositing.

Source/core/rendering/RenderLayer.cpp

Index: Source/core/rendering/RenderLayer.cpp
diff --git a/Source/core/rendering/RenderLayer.cpp b/Source/core/rendering/RenderLayer.cpp
index 9c306c9dd320254c78ad2ed96a6a0be03fe7c60c..de7234cdb0d11680e679e78f1793bed94151fa49 100644
--- a/Source/core/rendering/RenderLayer.cpp
+++ b/Source/core/rendering/RenderLayer.cpp
@@ -1491,7 +1491,14 @@ void RenderLayer::collectFragments(LayerFragments& fragments, const RenderLayer*
 
     // Shift the dirty rect into flow thread coordinates.
     LayoutPoint offsetOfPaginationLayerFromRoot;
-    enclosingPaginationLayer()->convertToLayerCoords(rootLayer, offsetOfPaginationLayerFromRoot);
+    if (rootLayer != enclosingPaginationLayer() && rootLayer->enclosingPaginationLayer() == enclosingPaginationLayer()) {
+        // The root layer is inside our pagination layer.
+        // FIXME: more work needed if there are nested pagination layers.
+        rootLayer->convertToLayerCoords(enclosingPaginationLayer(), offsetOfPaginationLayerFromRoot);

[mstensho (USE GERRIT), 2014/10/06 14:52:58]

This isn't really going to work right now (unless you're in the first column).
convertToLayerCoords() gives you the visual offset in the multicol container,
not the flow thread single-strip logical coordinates, which I was sooo certain
that it did, that I didn't even bother to double-check.

[mstensho (USE GERRIT), 2014/10/06 14:55:22]

Done.

With the latest patch, where I remove a hack from RenderLayer::location(), this
is now the right thing to do.

[Julien - ping for review, 2014/10/06 22:12:32]

Could we add some visual testing for the second column case as the original
change broke it?

[mstensho (USE GERRIT), 2014/10/07 14:55:35]

Done.

But then I had to fix even more. :-P It turned out that the documentation of
"dirty rect" was wrong, and therefore my fix was wrong too. Now it should be
right.

+        offsetOfPaginationLayerFromRoot = -offsetOfPaginationLayerFromRoot;
+    } else {
+        enclosingPaginationLayer()->convertToLayerCoords(rootLayer, offsetOfPaginationLayerFromRoot);
+    }
     LayoutRect dirtyRectInFlowThread(dirtyRect);
     dirtyRectInFlowThread.moveBy(-offsetOfPaginationLayerFromRoot);