We need to determine whether the problem is with tk/tkinter or specifically with IDLE, and whether it has anything with using ttk.  The attached file duplicates the skeleton of an IDLE editor in 14 lines of tkinter code.  This includes putting a ttk scrollbar on the tk frame.  It works for me on Windows.  If it misbehaves on Mac, uncomment ', Scrollbar' on the ttk line to get a ttk scrollbar on a ttk frame.  Also comment out the entire ttk line to get a tk scrollbar on a tk frame.