Did some trouble shooting.
1. With SCALE GUI ticked or not ticked, the problem persists.
2. Pixel Density is set to 1.4
If I change the desktop resolution to 1920x1080, and I launch DCS in VR (Oculust Rift), then the mouse can click anywhere in the DCS window. If I launch at native screen desktop resolution of 3840x2160, then the bottom half of the DCS window is unclickable.
Here is a catch, if I set desktop resolution to 1920x1080, THEN launch DCS in VR, THEN, once DCS is launched, I change the desktop resolution BACK to 3840x2160 while keeping DCS running, then whole DCS window is still clickable. The key is that it has to be 1080 or less when DCS launches, and after launching it, you can change the resolution to higher. Obviously, the DCS window will be smaller since it was launched at a lower resolution.
Hopefully that helps.