How to prevent window title bar height changes when app is maximized

[Sinspin]

Hey,

I don't think that you can prevent that, it's a Windows behaviour.
If you look for a workaround, one solution would be to align your client area to top or the bottom of the form to make sure the gap is not interfering. (Top align is the default behaviour)
Or, you draw the whole content by yourself, inclusive the window title bar.