I believe this change may have broken responsiveness on English Language & Usage when someone edits a post.
I'm not entirely sure, because I don't edit on EL&U as often as I do on ELL, but I have edited posts on EL&U on my phone before and I don't remember it being such a chore. The site is responsive when I'm just reading, so it's I didn't accidentally switch responsiveness off (the bottom of the page still has links to switch to "Mobile" and to "Disable responsiveness").
This happens on my iPhone in both Safari and the DuckDuckGo browser. It happens on my Mac in both Firefox and Safari. Also in Firefox on Windows. I don't remember it happening prior to the font change (I had edited several posts on EL&U on May 7th).
Editing an answer on EL&U, which I believe has a customized design:
Editing an answer on ELL, which is using the same sans-serif font as the rest of the network:
