TWO REVIEW FINDINGS, both real, and neither reachable from the existing
tree tests.
1. `item` WAS EFFECTIVELY REQUIRED. `render` writes `line_to_item[n] =
row.item`, so that map is SPARSE whenever a row omits the optional
`item` — and `seat_cursor` took `#` of it. A display-only tree (a
grouping node with `on_visit` unused, which the API explicitly allows)
made that length 0, so the cursor never left the header, TAB found no
row, and folding was unusable. It now counts visible rows explicitly.
The old tests could not catch this because every one of them supplies
`item`: under the reverted fix `tr_5` fails `left: 0 / right: 1` while
`tr_1` still passes.
2. THE ID CONTRACT WAS TWO CONTRACTS. The docs said "opaque, compared
by equality". Selection does compare with `==`, honouring `__eq` — but
collapse state stores ids as TABLE KEYS, and Lua indexes tables by raw
identity, consulting no metamethod. So a table id would satisfy one
half and quietly fail the other: after a refresh minted fresh id
tables, the cursor would be restored and the fold silently lost. A
divergence that shows up as a missing fold, arbitrarily later, with
nothing pointing back at the id.
Narrowed rather than generalized. Equality-aware collapse lookup is the
alternative and it is worse: `hidden_by_ancestor` runs per row, so it
turns a linear render quadratic to support a key type no consumer has
asked for. The contract is now the one both halves can honour — string
or number, compared by value — enforced by `check_ids` where rows enter
(`open` and `refresh`), so a bad id is a named error at the call site
instead of a lost fold much later. Q#TR3 in the framing records the
narrowing and why.
Verified: fmt, clippy, diff-check, --lib 1897/0, crdt 2082/0, listview
24/24, m4 150/0, gpu 221/0. Both fixes bitten independently.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Levi Neuwirth
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