Fix pixel snapping issue when transforming

Source/core/layout/Layer.cpp

Index: Source/core/layout/Layer.cpp
diff --git a/Source/core/layout/Layer.cpp b/Source/core/layout/Layer.cpp
index 9cc742b9c67879ea25f09b6ae2821e83c6918222..29ca4543d9cdf08860de74ac48f9a5a00abd7187 100644
--- a/Source/core/layout/Layer.cpp
+++ b/Source/core/layout/Layer.cpp
@@ -1627,6 +1627,11 @@ void Layer::collectFragments(LayerFragments& fragments, const Layer* rootLayer,
         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);
 
@@ -1634,7 +1639,7 @@ void Layer::collectFragments(LayerFragments& fragments, const Layer* rootLayer,
         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.