After more investigation, I found that my original speculation about a Cmd-M conflict with Cocoa Tk 8.5 was not correct. And the problem is not just limited to the keyboard accelerator for the Open Module command; it can also be seen with the keyboard accelerators for the Go To Line and New Ident Width commands, although none are always repeatable.  What all three have in common is that they use the tkinter.simpledialog module to get user input in response to the command. There does not seem to be a problem ever if the menu commands are clicked on rather than opened with a keyboard accelerator nor is the problem seen when using the older Carbon Tk 8.4.  The problem occurs at the end of the common Dialog class (simpledialog.py lines 167-169) where the tk "wait visibility" command does not cause the intended window to appear and the tk "wait window" (to be destroyed) command hangs waiting on a non-visible window.  Adding some debugging code makes it seem that the hangup is the display of the Entry widget within the dialog window.  Why that is a problem when going though the lengthy execution path triggered by a menu keyboard type-in, and not the clicking-on-menu-item path, is still not clear.  Most likely it is a bug in Cocoa Tk 8.5 but so far I have not yet been able to reduce it to a simpler, reproducible test case.

For 3.2rc2 and 3.2, I propose to add a temporary patch to IDLE that removes the keyboard accelerators for these three commands when running with Cocoa Tk 8.5.  That should prevent users from running into the hang and, thereby, losing work.  The patch should be available later today.