[New Multicolumn] Correct paint rectangle calculation for compositing.

Source/core/rendering/RenderLayer.cpp

 /*
  * Copyright (C) 2006, 2007, 2008, 2009, 2010, 2011, 2012 Apple Inc. All rights reserved.
  *
  * Portions are Copyright (C) 1998 Netscape Communications Corporation.
  *
  * Other contributors:
  *   Robert O'Callahan <roc+@cs.cmu.edu>
  *   David Baron <dbaron@fas.harvard.edu>
  *   Christian Biesinger <cbiesinger@web.de>
  *   Randall Jesup <rjesup@wgate.com>
@@ skipping 1473 matching lines @@
     ClipRect outlineRectInFlowThread;
     clipper().calculateRects(paginationClipRectsContext, PaintInfo::infiniteRect(), layerBoundsInFlowThread, backgroundRectInFlowThread, foregroundRectInFlowThread,
         outlineRectInFlowThread, &offsetWithinPaginatedLayer);
 
     // Take our bounding box within the flow thread and clip it.
     LayoutRect layerBoundingBoxInFlowThread = layerBoundingBox ? *layerBoundingBox : physicalBoundingBox(enclosingPaginationLayer(), &offsetWithinPaginatedLayer);
     layerBoundingBoxInFlowThread.intersect(backgroundRectInFlowThread.rect());
 
     // 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);
 
     // Tell the flow thread to collect the fragments. We pass enough information to create a minimal number of fragments based off the pages/columns
     // that intersect the actual dirtyRect as well as the pages/columns that intersect our layer's bounding box.
     RenderFlowThread* enclosingFlowThread = toRenderFlowThread(enclosingPaginationLayer()->renderer());
     enclosingFlowThread->collectLayerFragments(fragments, layerBoundingBoxInFlowThread, dirtyRectInFlowThread);
 
     if (fragments.isEmpty())
         return;
@@ skipping 1346 matching lines @@
     }
 }
 
 void showLayerTree(const blink::RenderObject* renderer)
 {
     if (!renderer)
         return;
     showLayerTree(renderer->enclosingLayer());
 }
 #endif