The GitLab layout is absolutely awful in its default state on smaller screens
Edit: I should probably have put this more clearly: obviously this is how I feel about it. As you can see below, I suspect I'm not the only one but I'm not a designer and I didn't do user tests with lots of people to confirm others share this view. Therefore, keep that in mind while reading the ticket.
The GitLab layout is absolutely awful in its default state on smaller screens. I have tried to point this out multiple times before e.g. with explaining that the new left sidebar collapses way too early, but this was in total pretty much ignored.
Therefore, here this demonstration of GitLab as it opens up per default, and GitHub as it opens up per default on a same smaller-width window size:
I think the comparison speaks for itself: while pretty on larger widths, the GitLab layout opens up in an absolutely unusable, chaotic horrible mess at this smaller width and I really think you should rethink completely how this is supposed to work at smaller screen widths.
(and no, not everyone always browses in fullscreen, so this might affect more people with modern larger screens than you think. And sure, I can always go fullscreen when encountering such a page, but the first impression is important and it is awful)