Found the first bug. Well, it’s not a bug, really, but it’s a problem. Spec says position:relative is meaningless on table-ish elements, so my rows and columns cannot reliably create a positioning context as intended.
(Also a weird thing happens just below the breakpoint: the
.wrap stops filling the whole height of the
[at-grid-row] for a while).
I can solve the breakpoint issue by making the rows
block below the breakpoint. I may just live with the positioning context issue. Things that need it typically already have their own wrapper…