fix(chat): keep the slash command selection visible and valid on long or filtered lists

[FelipeRamos-neuro]

Summary

Fixes keyboard navigation of the slash / @ command popover on long or filtered lists.

Closes #498

Problem

1. The popover scrolls when it has more commands than fit, but the arrow keys only moved the selection, so the highlight walked out of view. The rest of the list was reachable only with the mouse wheel, and ArrowUp from the first row wrapped to a hidden last row.
2. The selected index outlives the list it was chosen from. Arrow down to row 20 of 33, then type something that leaves one match: the index points past the end, no row is highlighted, and Enter calls applyPrefixSuggestion(undefined). That throws (Cannot read properties of undefined (reading 'startsWith')) and swallows the keypress. This is the same class of problem as the "Enter with an unmatched @ or /" fix (fix(chat): let Enter send a message that starts with an unmatched @ or / #464).

Solution

• scrollSelectedSuggestionIntoView scrolls the selected row into view with block: 'nearest' whenever the selection changes, including on wrap-around. nearest scrolls by the smallest amount and does nothing when the row is already visible.
• filterPrefixSuggestions resets the selection to the first row, since a narrowed list is a different list.
• activeSuggestionIndex(selected, count) returns the index if it still points at a row and 0 otherwise. Enter, Tab, both arrow keys, the highlight class and the scroll effect all read the index through it, so no other path that shrinks the list can leave it dangling.

Two commits: the scroll fix, then the stale-index fix.

How to test

I checked this by hand in JupyterLab, on main and on this branch, with a small test extension that adds a participant @many with 30 commands, so the popover has 33 rows in Ask mode. The full extension source and steps are in #498. In short:

• Scroll: type /, press ArrowDown 20+ times. On main the highlight leaves the visible list; on this branch it follows the selection, including the wrap from last to first and back.
• Stale index: type /, ArrowDown about 20 times, type cmd30, press Enter. On main nothing is highlighted, Enter does nothing and the console shows the TypeError. On this branch the single match is highlighted and Enter completes it.

Both symptoms reproduce on main and are resolved on this branch.

Testing

• tests/ts/chat-prefix-suggestions.test.ts covers scrollSelectedSuggestionIntoView (only the selected row is scrolled, wrap-around, missing popover / out-of-range index / no scrollIntoView) and activeSuggestionIndex (valid, out-of-range, negative and empty lists, and the exact narrowed-list scenario). If the helper returned the raw index, 3 of the new tests fail.
• The scroll behaviour was also checked in headless Chrome with the real base.css layout: with the fix, no selected row left the popover in any of the sizes tried (0 rows off-screen, against 26, 36 and 4 without it).
• jlpm jest (583 passed), jlpm tsc --noEmit, jlpm lint:check and pytest tests/ (2221 passed) are clean.

Not covered by an automated test: the reset in filterPrefixSuggestions itself, because there is no component test harness for ChatSidebar. The helper is the safety net for it, and the manual test above exercises the reset.

Not checked: Firefox, macOS, and Claude or ACP mode. Their command lists are built from a different source, but they use the same popover.

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