editing a file living in a GitLab repository within a browser window (“Edit File”), trying to display the changes between the working version and the last committed one

MediaWiki lets me do that, GitLab does not.

If you switch back and forth between several tasks, sometimes you need a tool showing you the differences between the original text (you started with) and your current text.

If I try to abandon such an editor window (resp. browser tab), MediaWiki asks me, whether I really want to do so. I like that very much.