Lexical version: v0.36.2+dev.esm (Playground)

Steps To Reproduce

1. Open the Lexical Playground
2. Enable mentions (@) and type the first letter to open the suggestion list.
3. Move the cursor to the last entry (Arrow Down).
4. Keep typing quickly so the list refreshes and shrinks before selectedIndex is reset. You can try with 'pooja'.
5. Quickly press Arrow Down again

Or for a better success rate at reproducing it, have Hammerspoon doing it:

hs.hotkey.bind({"ctrl", "alt"}, "p", function()
    hs.eventtap.keyStrokes("@")

    hs.eventtap.keyStrokes("p")

    hs.eventtap.keyStroke({}, "down")
    hs.eventtap.keyStroke({}, "down")
    hs.eventtap.keyStroke({}, "down")
    hs.eventtap.keyStroke({}, "down")

    hs.timer.usleep(1000000)

    hs.eventtap.keyStrokes("ooj")

    hs.eventtap.keyStroke({}, "down")
end)

The current behavior

Crash in the browser console

If I'm not mistaken, this is happening here:

The expected behavior

The highlighted index should reset (or short-circuit) when the list shrinks so we never dereference a missing option.

Impact of fix

Prevents a crash under fast typing in typeahead/mentions flows used by any app relying on LexicalTypeaheadMenuPlugin