A rung compacts when a claim is released, so an overlay's z can move without it
doing anything. Six of seven call sites read z once at open time and kept a
number that was later handed to the next overlay: two of them at one level, DOM
order deciding which wins and which eats the clicks. Reproduced with two menus
and a third opened after closing the first -- both ended up at 501.
claimFor(rung, node) binds the node, so there is nothing left to forget, and the
raw claim() stays for the one case with no node. The test walks the registry
rather than naming overlays, because the bug was one site remembering and six
forgetting.
Also drops a no-op ternary that picked the modal rung either way.
digital-garbage
73e81b0263