I also don't think an undecorated window would increase the performance noticable compared to it stretched to fullscreen. At least it made no difference in my tests.
This is something that I do think that I've seen (especially if the computer allows only a limited number of full-screen resolutions).
The first thing I do when starting a game that opens in a window is to look for a way to make it fullscreen.
That's fair--but I do want a windowed mode for those who want to have one.
Ah, you're probably thinking of non-captured fullscreen.
Possibly, but I'm no confident of that; I feel like I have seen a desire for an actual windowed mode that is undecorated.
With all that said, however, you two do make some good points in favour of decorations in windowed mode, and I'm thus convinced that it's likely better to go with a decorated mode. I'll do that then, I think!
Thank you both for your help! ^_^