`mov` accumulates a delta onto the slot and `reposition` overwrote it, and
both legitimately land on one widget in one frame: `List::place`'s
Bottom-known branch offers a row a same-size box that has moved (`mov`),
then corrects the placement inside it when the row's cached height no
longer matches what the row reports (`reposition`). That is what a wrapped
transcript row hit, and what the `move_applied == ZERO` debug assert was
standing in for -- an assert against a case that happens is not a
guarantee, it is a crash.
The slot means `move_applied + repositioned` now, both halves recorded on
`ActiveData`, so `reposition` adds the move rather than dropping it and
stays idempotent. The assert it replaces is a `debug_assert_eq!` that the
slot still holds that sum on entry -- i.e. that nothing but those two ever
wrote it.
Test: `a_widget_moved_by_its_parent_and_then_placed_inside_it_lands_at_the_placement`,
which draws the child at the offered position (-100px) rather than the
placement (100px) without the fix. Verified against the `.wrap(true)`
repro from docs/IRIS_TODO.md (draws correctly, no panic) and an emulator
bench run with assertions live.
iris
f5b88932b4