I have a TCL C6 on Oreo and videos on YouTube slightly exceed the screen. There's an option to change the aspect ratio but videos still appear to be cropped. When searching the web, the common suggestion is to disable overscan but it also doesn't help. I think it only applies to external inputs. I've tried Smart YouTube Player but it has the same issue and seems to buffer a lot more than the official app. I can live with it but there might be a simple solution I'm not aware of.
I'm having the same problem with my TCL C2. It's especially annoying when watching game streams where HUD elements are clipped at the edge of the screen.