Autocomplete selection jumps erratically when pressing up/down arrows and sometimes starts at the wrong position.

Reproduction

1. Start a session (send any message)
2. Type / to open autocomplete
3. Navigate with arrow keys and watch the highlighted row jump around spasmodically

Only happens with an active session, which adds /undo, /redo, etc. to the list. I also have the EXA MCP server configured, which adds additional slash commands and may be necessary for reproducing the issue.