Get center of viewport or screen inaccurate?

I’m trying to rotate / angle a widget, or any of it’s children towards the mouse, the widget, and it’s children are positioned in the center of the screen.
The problem i’m facing is, there doesn’t seem to be a reliable way to get the centre of the viewport or the screen, not even sure which one to use, but at this point it doesn’t really matter, after debugging the values returned from relevant functions, I’ve never successfully got the center of the screen.

and it feels like widgets use completely different coordinates altogether.

I’m missing some fundamental understanding of how the view port / screen is constructed and how to interpret the data correctly.
Can someone help shed some light on what i’m missing? Cheers