The Edit Recurrence dialog comes with a pretty silly UX aimed at offering users the ability to see the highlighted dates based on the their recurrence preferences.

What's wrong

• The minimonth used in this dialog doesn't get the new style.
• This dialog adds extra months whenever it's resized and then it stores the new size int he XUL store.
• After extra months are added is impossible to make dialog smaller.
• The tests covering this area need to do some sketchy await in order to pass in mochitest.

What to do

• Make sure the style it's consistent.
• Only show 3 months at all times.
• Add a left and right arrow to navigate through the months.
• Update the tests: https://searchfox.org/comm-central/rev/07e2b09826d048a1f25e7cc1f0a01f7a8120ce72/calendar/test/browser/recurrence/browser_customMinimonthCount.js#22

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.

That the number of months change depending on size... I think we could keep it. When it was broken we did get a bug report that someone was depending on it, and for longer recurrences it does make some sense wrt planning.

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.

Drive-by comment: Note that the bold day numbers only appear for the first month.

This is currently working in my active patch for bug 1805938. I'm hoping to get a patch ready for review in the next few days.