As identified in Issue22168, the changes introduced here do not work when the user supplies a Canvas rather than relying on the turtle default.  There is a patch there that should solve that issue.  But I now think that the original approach was not an ideal solution.  For one, it introduced Tk-specific code into TurtleScreen, which is supposed to be independent of the underlying graphics toolkit.  Two, as we now know from subsequent work, though the setting of the -topmost attribute does move the window to the front, it does not change which gui application has the keyboard and menu focus.  So the user still needs to click on the now-raised window to interact with it.  The code added to Lib/turtledemo/__main__.py in Issue17172 solves that problem through the use of a little Applescript.  There are a number of turtle and turtledemo changes pending at the moment.  It might be worthwhile to re-examine this (perhaps, as earlier suggested, try to deal with it at the tkinter level through an option) after the dust settles.