Copilot's sixth pass; both correct, and the second is a front-end
divergence of exactly the kind this PR exists to remove.
**The high-water mark was committed only when the extent changed.** A group
count that rose while a header line fell by the same amount left `next.rows`
equal, so the larger count was discarded and a later repaint under-reserved
— the DOM-scroll bug again. `frameCost` is now assigned unconditionally and
only `sendViewport()` is gated on the extent moving.
**`col_groups` started a new run when `collapsed` differed; the TUI does
not.** `src/ui/grid.rs` breaks a run on `group_name != prev_group` alone.
Collapse is category-scoped, so two adjacent groups from different
categories may legally share a name and differ in it, and the browser would
then have drawn a second indicator the TUI suppresses. Coalesced by name,
with the run taking its first group's indicator, as the TUI paints it.
`column_groups_coalesce_by_name_as_the_tui_does` pins it; restoring the
`(name, collapsed)` key splits the span and fails.
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Edward Langley
fb2122f23d