Right now the tiling paint and draw code expects that checking whether a tile exists or not is a synchronous operation. We need to pass invalidation rects for tiles that have been invalidated or have never been filled from the compositor thread back to the webkit thread for painting. The compositor thread should also be robust to not having a tile when it needs it and may need to either block or checkerboard.
https://bugs.webkit.org/show_bug.cgi?id=64772 takes care of this in a somewhat roundabout way by changing the semantics of when tiles can get evicted.
This is fixed now via the TiledLayerChromuim/CCTiledLayerImpl changes.