Hi Ned. Here is one more attempt, hopefully the final one. I tested with IDLE on 10.14 (VM), 10.15(hard), 11(hard), 12(VM). I used Python 3.10.0. I replaced libtk8.6.dylib with the Tk lib compiled from the tip of the macosx_filedialog branch. I could not reproduce any of the issues that you reported. (But I hope you will retry as well, since just replacing the tkMacOSXDialog.c file in 8.6.11 is not the same as using the soon-to-be 8.6.12 release candidate.)
I will attach the tkMacOSXDialog.c that I used.