A small userscript for collapsing all files on a GitLab merge request diffs page. Can be used by either Greasemonkey for Firefox or Tampermonkey for Google Chrome.
With Tampermonkey installed, simply navigate to the raw user.js file and it should prompt you to install the script. You should of course inspect the JS first to add your personnal gitlab servers and ensure it doesn't do anything nasty.
Mad arrow drawing skills