"Setup was successful" dialog box text overflows the box, and last line can't be read.

+1. I observe it too.

So it'll take a bit more rearranging to get everything to fit - I've attached one proposal that shuffles the thanks to Mark Hammond down below the "enable long path names" button. (It's grey text in the screenshot, but I've switched it to black already.)

Alternatively, we could drop that note completely. It's entirely possible that Mark feels he's been thanked enough, and is now just embarrassed that we keep it around :) There are certainly enough other people with significant contributions on Windows who also deserve thanks.

One final option would be to move the "fix long paths" button to earlier in the install, though I'm not doing that during RC. The reason it's not part of the regular install is because it always requires Admin, and while it would be great to have it enabled by default, we already have enough people getting confused about whether admin is required to install or not (it's not, but you have to change the default launcher option on the front page).

Anyone have any thoughts?

Yes, while I appreciate the gesture, I am somewhat embarrassed these days - as you say, many others deserve this more than me these days.

So please remove it.

Thanks, Mark. We shall not hold you solely responsible for the port any longer ;)

New changeset 6444ca946984c638c67a72aac22fd6d3cc650c16 by Steve Dower in branch 'master':
bpo-41526: Fixed layout of final page of the installer (GH-21871)
https://github.com/python/cpython/commit/6444ca946984c638c67a72aac22fd6d3cc650c16

New changeset 75c2281762932c4e5f0bd7deae40ef014f704de2 by Miss Islington (bot) in branch '3.9':
bpo-41526: Fixed layout of final page of the installer (GH-21871)
https://github.com/python/cpython/commit/75c2281762932c4e5f0bd7deae40ef014f704de2

Fixes merged, just need Łukasz to confirm it'll be in 3.9.0 and we can resolve this.

Thanks for the report! This should be in the next 3.9 RC.

:-)