After hours of failure to understand crazy 'impossible' behavior, I went back to 'timing issue' and this code.

    def run_module_event(self, event):
        if macosx.isCocoaTk():
            # Tk-Cocoa in MacOSX is broken until at least
            # Tk 8.5.9, and without this rather
            # crude workaround IDLE would hang when a user
            # tries to run a module using the keyboard shortcut
            # (the menu item works fine).
            self.editwin.text_frame.after(200,
                lambda: self.editwin.text_frame.event_generate(
                        '<<run-module-event-2>>'))
            return 'break'
        else:
            return self._run_module_event(event)

When I used the Run menu instead of F5, the problem went away.  The problem also went away when I added 'and 0' to the condition above to disable the workaround.  3.8 also works fine with the workaround disabled.  Ned, if you verify enough to agree, I will get rid of the wrapper and remove the leading '_' from the real handler.