Problem:
Legacy cmdwin set 'buflisted', but new one doesn't.
https://github.com/neovim/neovim/issues/40431#issuecomment-4811593353
'buflisted' useful for:
- "bufferline" style tablines, they usually show only listed buffers.
- some automatic actions can be conditioned on whether a buffer is
listed; 'buflisted' signals that the buffer is "important enough".
Solution:
Set 'buflisted'.