[New Multicolumn] Clip correctly if the "root" layer is inside the multicol container.

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 1547 matching lines @@
     clipper().calculateRects(paginationClipRectsContext, LayoutRect::infiniteIntRect(), 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());
 
     // Make the dirty rect relative to the fragmentation context (multicol container, etc.).
     RenderFlowThread* enclosingFlowThread = toRenderFlowThread(enclosingPaginationLayer()->renderer());
     LayoutPoint offsetOfPaginationLayerFromRoot; // Visual offset from the root layer to the nearest fragmentation context.
-    if (rootLayer->enclosingPaginationLayer() == enclosingPaginationLayer()) {
+    bool rootLayerIsInsidePaginationLayer = rootLayer->enclosingPaginationLayer() == enclosingPaginationLayer();
+    if (rootLayerIsInsidePaginationLayer) {
         // The root layer is in the same fragmentation context as this layer, so we need to look
         // inside it and subtract the offset between the fragmentation context and the root layer.
         offsetOfPaginationLayerFromRoot = -rootLayer->visualOffsetFromAncestor(enclosingPaginationLayer());
     } else {
         offsetOfPaginationLayerFromRoot = enclosingPaginationLayer()->visualOffsetFromAncestor(rootLayer);
     }
     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.
     enclosingFlowThread->collectLayerFragments(fragments, layerBoundingBoxInFlowThread, dirtyRectInFlowThread);
 
     if (fragments.isEmpty())
         return;
 
     // Get the parent clip rects of the pagination layer, since we need to intersect with that when painting column contents.
     ClipRect ancestorClipRect = dirtyRect;
-    if (enclosingPaginationLayer()->parent()) {
-        ClipRectsContext clipRectsContext(rootLayer, clipRectsCacheSlot, inOverlayScrollbarSizeRelevancy);
+    if (const RenderLayer* paginationParentLayer = enclosingPaginationLayer()->parent()) {
+        const RenderLayer* ancestorLayer = rootLayerIsInsidePaginationLayer ? paginationParentLayer : rootLayer;
+        ClipRectsContext clipRectsContext(ancestorLayer, clipRectsCacheSlot, inOverlayScrollbarSizeRelevancy);
         if (respectOverflowClip == IgnoreOverflowClip)
             clipRectsContext.setIgnoreOverflowClip();
         ancestorClipRect = enclosingPaginationLayer()->clipper().backgroundClipRect(clipRectsContext);
+        if (rootLayerIsInsidePaginationLayer)
+            ancestorClipRect.moveBy(-rootLayer->visualOffsetFromAncestor(ancestorLayer));
         ancestorClipRect.intersect(dirtyRect);
     }
 
     for (size_t i = 0; i < fragments.size(); ++i) {
         LayerFragment& fragment = fragments.at(i);
 
         // Set our four rects with all clipping applied that was internal to the flow thread.
         fragment.setRects(layerBoundsInFlowThread, backgroundRectInFlowThread, foregroundRectInFlowThread, outlineRectInFlowThread);
 
         // Shift to the root-relative physical position used when painting the flow thread in this fragment.
@@ skipping 1304 matching lines @@
     }
 }
 
 void showLayerTree(const blink::RenderObject* renderer)
 {
     if (!renderer)
         return;
     showLayerTree(renderer->enclosingLayer());
 }
 #endif