Isn't AMP for email kinda what you are describing?
"The AMP version of the message is embedded into the email as a new MIME part, in addition to the HTML and plaintext, ensuring compatibility across all mail clients"
We prefer 990px containers where possible, so if you're using gitlab on a browser smaller than that it'll usually butt up against the edges.
Couple of questions about your issue:
1. Any pages this is particularly bad for? Some dense pages (e.g. Pipelines or side-by-side diffs) need all the width they can get
2. what screen size do you normally use?
3. In Settings > Preferences > Behaviour, are you using Fixed layout or Fluid? Fixed should give you left/right margins if your window is >1280px
4. Do you collapse the left/right sidebars? I usually keep them both collapsed which helps reduce the crowding. But since they are expanded by default this is not an ideal solution
"The AMP version of the message is embedded into the email as a new MIME part, in addition to the HTML and plaintext, ensuring compatibility across all mail clients"
From https://amp.dev/documentation/guides-and-tutorials/start/cre...