Nice insight, Serhly!  The patch addresses the menu concern I brought up in msg223631 of Issue21933.  I evaluated the patch with all three OS X Tk variants (Cocoa, Carbon, and X11) and it looked good to me.  The only review comment I have is that ideally, with Cocoa or Carbon Tk's, the "About ..." menu item in the "Apple" menu row would be customized to display the "aboutTurtleDemo" window; as it stands, it defaults to a Tk-supplied "about Tk" window.  The problem with doing that is that there are Tk-variant-specfic (Cocoa vs Carbon) ways to do that and it shouldn't be done at all for X11 so the implementation would get a bit messy;  the relevant code for IDLE is in macosxSupport.py towards the end of overrideRootMenu().  It's probably overkill for turtledemo so I'm OK with the patch as it stands (note I did not try it on any other platform).