This is a regression from my layout/visual overflow changes from a while back. Layers used to incorporate all forms of overflow into their size, and now they don't. Either visual overflow needs to be incorporated, or the code in calculateRects that looks only at box shadows should be expanded to include all visual overflow.
Actually never mind, this works fine. However it should probably be patched to just use visual overflow for the future, so that outline works right once it becomes part of that.