I noticed this on a macOS 11.0.1 box: When I run "python -m test.regrtest -uall" the IDLE testsuite is run ('test_idle'), and those tests pop up windows and won't make progress until I close those windows.

Strange.  test_idle runs fine on my Mohave, so I cannot investigate what you see.  Tests of classes that create windows and call wait_window pass _utest_=True and wait_window calls are guarded by "if not _utest:".  I believe this is always in __init__.  You could grep idlelib for _utest, find a window that stays open, and add prints to see what tkinter call is causing the hang, and if it is in the not-_utest suite.

When I run the entire suite rather than just test_idle, I instead get multiple "Python quit unexpectedly" boxes and failures in 5 other tests.

Ned, do you have any experience of tk acting so differently on different macOS versions?

This is the behavior seen with Tk 8.6.8 *built* on macOS 11.0 Big Sur; I'm assuming that's what Ronald was testing. We'll be updating the macOS installers to use a more recent Tk for installers built on 11.0 and this should go away. Note that Tk 8.6.8 *built* on, say, 10.9 as in the current python.org installers does not exhibit this behavior when *run* on 11.0.

I was using a Tk 8.6.8 build on macOS 11. Looking at this more closely there are clearly other problems with that build: All windows that were created by the test were completely black except for the window chrome. 

@terry.reedy: I got those crash reporter pop-ups as well, they seem to be related to tests where the interpreter is made to crash as part of the test (for example in the concurrent.futures tests).  I hope we can find a way to suppress those pop-ups.