That’s telling you what the difference between the bounds and the content area is. In other words, that’s the height of the Title Bar. As far as I can tell, 50 is a magic number you chose. Perhaps at random?
If you set the Window.Bounds.Top
to DesktopDisplay.AvailableTop
the Window will be correctly positioned under the Menu Bar. Is that not what you are trying to do?
Please review the thread TomE linked.