I have just added another splitter and the splitterxOnIdle is called very time the mouse enters the main frame, which confirms my suspicion, but at this stage and with my current understanding of ...

I don't know why you're still seeing the attrdict3 error, but wxPython 4.2.0 isn't going to work with Python 3.11 anyway. I'd suggest using an older Python version, or you can use one of the wheels ...