In MSFS sim update 5 (SU5), the white triangle of the menu bar on top of the screen doesn’t hide itself as it did in the past when you move away the mouse from it.  Maybe a bug and probably will be fixed in future updates.

I remember there is a command or a key or something that can hide it but I don’t remember what it exactly is.

However, I do remember there is an addon I downloaded a few months ago that can hide the menu bar.  It’s called No Handle Bar from ither.

I didn’t use it then.   Now a good temporarily fix or maybe for good.