I can somewhat reproduce the problem with both IDLE and tkinter.Tk() in 3.9b3 and an external monitor.

What I did:
- Start IDLE (or a shell running tkinter.Tk()
- Maximize window without changing monitor settings: OK
- Revert to regular window
- Rotate monitor 90 degrees in the Display settings (monitor itself not rotated)
- Maximize window again and revert back to regular size
- Revert changes to monitor settings

When I maximise the window with a rotated display the window grows to cover most of the display, but not all of it: There is a blank area between the menu bar and the top of the window. The height of that area is about that of the titlebar of the window. 


The window grows to the right size if I first rotate the screen and then start tkinter.Tk(). But then window won't go full-size when the screen is rotated back, some space is left between the righthand side of the window and the edge of the screen (again about the same amount of pixels as the height of the title bar).

Issues like this tend to be problems with the macOS port of Tk itself, I expect that to be the case in this instance as well. 


System:
- Python 3.9b3 (python.org installer)
- macOS 10.15.6