On Mac, hold down the "c" key and a little menu with accented versions (ç, ć, č) appears, with numbers to choose which one. The flip side is that holding down letters or numbers doesn't repeat them on a Mac.

My old mac muscle memory is telling me option+e, c.

I don't have one anymore to test though, but I used to use the mac layout on Linux for a while exactly for composing using alt.

Hmm, I get ´c when doing option-e c. I feel like it should work to combine the two, but ... Looks like that functionality only applies to the vowels (and not y): á´b´c´dé´f´g´hí´j´k´l´m´nó´p´q´r´s´tú´v´w´x´y´z.

I'm not seeing that behavior; my letter/number keys simply repeat. But I'm sure I've seen it before. I guess there may be an option somewhere to enable/disable it, but offhand I can't seem to find it.

Or maybe this has changed (back) in a recent OS update? I'm running 12.0beta on this machine.

It's been around since Lion or Mountain Lion, IIRC. There's a toggle in keyboard settings to choose between repeating or hold-for-alternate behavior (like how iOS works). The substitution behavior is the default, but if you already had repeat mode on it may have carried forward. The old way was option+e and a vowel for an accute accent mark, but I don't think that works with consonants.

Yes, I expected a toggle in the keyboard settings, but I can't seem to find one on Monterey. (What exactly is it labeled, and where located? Maybe I'm just being blind...)

The dead-keys like Opt-E for acute only work with a small set of letters in the standard US keyboard layout, although it's possible for a different layout to support more -- subject to the combinations existing as precomposed characters in Unicode. (For other combinations, you'd need to enter a combining accent after the letter, and rely on the font to supports placing it properly.)

The (Neo)Vim digraph is `C'`: Ctrl+k, Shift+c, '

That gives me a Ç

edit: Looks like it's supposed to work, I don't know what I'm doing wrong: https://help.ubuntu.com/community/GtkComposeTable

Are you in en_US? The sequence you mentioned should definitely work in that locale. Here’s a more authoritative link than the manually-updated wiki page: https://gitlab.freedesktop.org/xorg/lib/libx11/-/blob/master...

If you’re not in en_US you can find other locales and their Compose sequences in the nls directory.

what do you get using comma?

The same thing, ç

edit: hold on, that's only in Firefox. Anywhere else, comma+c does ç and apostrophe+c does ć