YES!!!! OS Caption Height of 23 solved the problem.
Man, that was sneaky. How in the word did you figure that one out? I'm normally pretty good at inspired-trial-and-error, but I'm not sure I would have stumbled upon that one for a long, long time.
OS Caption Height did, in fact, turn up in the side-by-side text compare in Notepad++, but I ignored it, figuring that Modern wanted a taller caption/titlebar than Captain, and having no idea it would impact the buttons.
I'm both impressed and grateful. Thank you.