This should already happen automatically:

The Control.NOTIFICATION_THEME_CHANGED happens first, then the class one. And is there where the theme is initialized, so the minimum size is out of date.

Thanks!

Now the size is updated multiple times.
The update in Control should be deferred or something.

Couldn't that potentially create problems elsewhere?

Yeah, it needs some deeper investigation.

Alright, I found a better solution: make the theme code be ran by the theme_changed signal instead, as that fires before the minimum size update. Will open a PR.