The bounded-rows test fails on main: 300 skill items mount 300 rows.
The scroll guard passes on main and pins that a highlighted row far
down the list is brought into view once the list is windowed.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

The `/`, `$` and `@` menu rendered every item, so a few hundred skills
mounted hundreds of rows to show ten and each keystroke re-rendered them.
Render rows through LegendList with Command's `virtualized` mode, as the
branch, model and command palette pickers do, and scroll the externally
highlighted row into view through the list ref. Unmounted rows are
brought into the window with scrollIndexIntoView, then nudged with native
scrollIntoView so the menu's scroll padding still applies.

Fixes #28489

Co-Authored-By: Claude Opus 5.5 <[email protected]>

LegendList measured the list before its rows gave it height, so the
first frame on open showed 4 rows in the full-height menu and the rest
appeared a few frames later. Reserve the rows' minimum height on the
list so the first measurement sees the real viewport.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

Filtering a scrolled list left the reset highlight off-screen, and
scrolling away unmounted the aria-activedescendant target. Pin the
active row with LegendList's alwaysRender (refreshed via dataVersion)
and let that row scroll itself into view when it becomes active,
replacing the scrollIndexIntoView path that no-ops after a data change.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

Only a window of options is mounted, so set aria-posinset and
aria-setsize from the full result list for assistive technology.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

The fade only updated on layout and scroll, so an overflowing menu
opened without its bottom fade until the first scroll. Update it when
rows are measured too.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

Main now labels skills with their own name, not a title-cased one.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

…hort windows

The list reserved its first-frame height with min-height, so a short
window could not shrink it and the clipped menu surface overflowed; each
row's scrollIntoView then scrolled that surface. Reserve the height as a
shrinkable flex basis and scroll the highlighted index through Legend
List, like the other virtualized pickers. Drop the visual-only fade and
first-frame tests.

AI-assisted; verified with the tests and tools listed above.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

The fixed flex basis assumed 28px rows, but skill rows with a source
badge are 30px and every row is 32px below the sm breakpoint, so short
lists got an inner scrollbar and a clipped last row. Reserve the rows'
minimum height only until Legend List has placed them, then let the list
size to its content like before, so a short window can still shrink it.

AI-assisted; verified with the tests and tools listed above.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

…fade

Legend List ignores class padding and CSS scroll-padding in its scroll
math, so a highlighted row scrolled into view ended 8px past the list's
clip edge, under the bottom fade. Give the list its vertical padding as
style padding and scroll with 24px viewport margins, as the command
palette does.

AI-assisted; verified with the tests and tools listed above.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

…changed

The highlight scroll effect depended on the items array identity, so an
equal new array (thread shells updating in the @ menu) jumped a scrolled
list back to the highlighted row. Key it on the highlight index and the
first item's id instead, which still re-scrolls when a filter changes.

AI-assisted; verified with the tests and tools listed above.

Co-Authored-By: Claude Opus 5.5 <[email protected]>