The code in that dialog is indeed a bit nonsense. I had actually reworked it in https://phabricator.services.mozilla.com/D139279 but haven't landed that yet since it has test failures on windows which I didn't yet get back to. If you want to steal it, feel more than welcome.
Ah nice, good to know.
We won't touch the dialog structure in this bug but simply tackle the minmonth implementation inside of it, so this body of work is a bit unrelated.
That the number of months change depending on size... I think we could keep it
I'm sorry but absolutely not.
That's the weirdest piece of UX I've ever seen and just because someone finds it useful it doesn't mean it's right.
Showing 3 months, and being able to switch months and navigate through the year is enough to allow users to see how their recurrence visually translates on the calendar.