Hi @hngbr,
Your Phone/Main monitor:
Resolution: 1600×720, Aspect Ratio: 2.22: 1
What you want:
Resolution: 800×600, Aspect Ratio: 4:3
You can’t stretch the game to fit your whole screen without getting those black bars because these two resolutions have different Aspect ratios.
If you want the game to cover your entire screen on your main monitor, you’ll need to adjust the game’s resolution to match your phone/main display’s aspect ratio, which is 2.22:1. So, instead of 800×600, we suggest picking a lower resolution with a 22.2:1 aspect ratio to make it fit better (e.g 800×360)
note: make sure to set spacedesk viewer resolution to your phone’s native resolution too.
since you are only using mirroring mode, just go to spacedesk viewer settings > Resolution > uncheck the ‘Use custom resolution’