Fix pixel snapping issue when transforming

Source/core/layout/Layer.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 1609 matching lines @@
         const Layer* 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);
     }
 
+    LayoutSize subPixelAccumulationIfNeeded;
+    if (isAllowedToQueryCompositingState())

[qiankun, 2015/03/10 10:13:16]

Check isAllowedToQueryCompositingState() to make sure compositingState() is
available, and make following layout test happy:
virtual/regionbasedmulticol/fast/multicol/hit-test-float.html
virtual/slimmingpaint/fast/multicol/hit-test-translate-z.html
virtual/slimmingpaint/fast/multicol/hit-test-float.html
virtual/regionbasedmulticol/fast/multicol/hit-test-translate-z.html

[leviw_travelin_and_unemployed, 2015/03/10 17:55:51]

This isn't the right fix.

I talked to chrishtr and we agreed that we should ensure that the document
lifecycle is advanced to CompositingClean prior to hit testing. That would avoid
the issue and assert and unblock your previous change. I suggest doing that in a
separate patch.

+        subPixelAccumulationIfNeeded = compositingState() == PaintsIntoOwnBacking ? LayoutSize() : subPixelAccumulation;
+    else
+        subPixelAccumulationIfNeeded = LayoutSize();
     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.
-        fragment.moveBy(fragment.paginationOffset + offsetOfPaginationLayerFromRoot);
+        fragment.moveBy(fragment.paginationOffset + offsetOfPaginationLayerFromRoot +  subPixelAccumulationIfNeeded);
 
         // Intersect the fragment with our ancestor's background clip so that e.g., columns in an overflow:hidden block are
         // properly clipped by the overflow.
         fragment.intersect(ancestorClipRect.rect());
 
         // Now intersect with our pagination clip. This will typically mean we're just intersecting the dirty rect with the column
         // clip, so the column clip ends up being all we apply.
         fragment.intersect(fragment.paginationClip);
     }
 }
@@ skipping 1304 matching lines @@
     }
 }
 
 void showLayerTree(const blink::LayoutObject* renderer)
 {
     if (!renderer)
         return;
     showLayerTree(renderer->enclosingLayer());
 }
 #endif