>> Comments/objections?
>
> I suggest to try this on a system that has a lot of fonts installed,

I don't have such a thing.  My `font-family-list` has only 1k entries.
But I tried it with `corfu-mode` on a slow build (debug +
enable-checking) and I did not see any lag even though it presumably
rebuilds the list after each character I type.

This said, clearly, there is room for improvement via caching, so the
new patch below does that.

> to make sure this doesn't slow down customize-face too much.

It should make no difference in general: the list of completions is
computed only when a completion is actually requested on that field.

> Also, the doc string of customize-face should be amended to say that
> it offers completion on fonts.

Hmm... the docstring currently doesn't describe the UI at all (and
I can't think of a reason why it should).  Similarly the manual doesn't
seem to describe any part of the UI of `customize-face`.
Apparently, we presume the UI is self-explanatory.  If we want to fix
that, I think it's beyond the scope of this patch submission.

> Btw, how does one invoke completion on the face attributes which are
> supposed to provide one, such as weight or colors?  M-C-i doesn't seem
> to do it for me, or what am I missing?

M-TAB worked for me on the "Foregound" color name field.
For weights, I see a menu rather than a text field so there doesn't seem
to be a "completion" option.


        Stefan