@Scott
Scott
There are big black bars while screen mirroring on GrapheneOS phone to a 23" 4K DELL monitor. How-to use all of the screen?
That's normal in screen mirroring mode, as the phone has a 20:9 aspect ratio, whereas the external monitor has a 16:9 aspect ratio, which is why it uses letterboxing on the external monitor.
There might also be some pillarboxing like black bar on on the left or right side, due to Android typically not rendering below the front (selfie) camera.
And:
Scott
Is there something other I can do?
Fullscreen should be achievable by using the desktop mode (instead of the mirrored mode):
And:
Scott
Even no 4K possible 👎, sadly only 1080p.
Which USB-C cable / docking station is being used?
In case the phone's USB-C port only supports DisplayPort 1.2 (unclear), then 4K @ 60Hz might only work with a USB-C docking station that has USB 2.0 downstream ports only (or with a direct USB-C to DisplayPort or USB-C to HDMI 2.0 cable), due to bandwidth constraints.