Hi Sebastian, I'm Graeme, an independent adviser here to help.
I'd firstly recommend updating your graphics drivers, as this can often fix issues like this.
AMD Radeon:
https://www.amd.com/en/support
NVIDIA GeForce:
https://www.geforce.com/drivers
Intel Graphics:
https://downloadcenter.intel.com/product/80939/...
Also check that your monitor is on the correct resolution by searching Start for Display Settings. Here you can change the resolution to the recommended if it's not already set correctly.
If none of the above helps, could you post a photo of the issue while in a game? That might help me find the cause of the problem. Thanks!
Note: Non-Microsoft websites are linked. The pages appear to be providing accurate, safe information. Watch out for ads on the site that may advertise products frequently classified as a PUP (Potentially Unwanted Products). Thoroughly research any product advertised on the site before you decide to download and install it.