Documented as macOS specific at https://www.tcl.tk/man/tcl8.6/TkCmd/wm.htm.  But I would not know about C-click from "Specifies the path of the file referenced as the window proxy icon (which can be dragged and dropped in lieu of the file's finder icon). "

IDLE document windows already have the absolute path in the title.  There is only an issue if it is too long for the space, such as when editing installed idlelib test files.  I don't know what will happen if/when you use tabbed windows.

In the meanwhile, it would be easy to add
    if long and sys.platform == 'darwin':
        # Only valid as long as 'long' is absolute path.
        self.top.wm_attributes("-titlepath", long)
to the end of editor.EditorWindow.saved_change_hook.

Ned or Ronald, is this worth it?  Is there any downside to this?