While 8.5.9 on OSX is more usable than 8.5.7, it is known to crash occasionally. Hence the startup warning. See idlelib.macosx.tkVersionWarning and https://www.python.org/download/mac/tcltk/. (The problems listed in the latter for 8.5.9 seems incomplete ;-). So I am not greatly concerned about another one specific to Apple 8.5.9.
runscript.Script_binding.run_module_event has a workaround, a 200 millesecond delay, when one uses F5 instead of the menu. Would that work here? (editor.get_accelerator has a different workaround for other keys.)