Screenshot testing / visual testing is mostly about different screen sizes (besides browser differences).
At the moment it isn’t possible to test multiple desktop screen sizes in the same test. This is an essential functionality, that should be included.
At the moment testing many different desktop screen sizes is at the best cumbersome and at the worst not realistic because several tests have to be set up & started, just to test a handful of different sizes.
Different desktop screen sizes would be a great addition, as it would speed up testing a lot. At the moment I would have to go through a lot of individual tests to make sure it works on various desktop screen sizes.