vertico buffer mode slow when font size is decreased

[rougier]

I've observed that when I decreased font size while in minibuffer (using C-x C--) with vertico-buffer-mode active, it makes navigation/typing to become really slow (it is not the case for increased font size). I would thus like to know if it is the case for everyone or only me.

[minad]

I don't see any slow down after I did M-x C-x C-- - - - on Emacs 29.1 Linux/X11. You can capture a profile and check where the slow down is coming from.

EDIT: I missed that you specifically mentioned vertico-buffer-mode. I observe a horrible slow down there too.

[minad]

Looks like a problem in the display engine in relation with face mapping, at least redisplay appears prominently in the profile. I suggest to avoid text-scale-adjust. I've seen moderate slow down caused by it before in other buffers, but not as bad as this one.

[minad]

Investigating a little bit more, this seems to be caused by the pre-redisplay function of vertico-buffer. At least the problem goes away when I remove this hook. I have to check if I am doing something problematic there which somehow causes a recursion of the redisplay code. Ideally such pre-redisplay functions are completely avoided, but I guess I didn't find a better solution back then when I wrote this.

EDIT: Worked around the problem in 37e01c1. This is definitely a problem in the display engine, where we hit a slow path for large vscroll values.

[rougier]

Thanks, I confirm it fixed the problem.