When we have tiling we fail the native surface checks in gfxWindowsNativeDrawing::BeginNativeDrawing and so we fallback to use dual pass rendering. This causes us to fail dom/plugins/test/mochitest/test_painting.htm and test_visibility.html Assuming we care about this regression, I think we can add an API to DrawTargetTiled to get the tile for a given rectangle in user space. If this rect only intersects with a single tile, then it can be returned. This would let us do efficient native drawing in the majority of cases.
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