Steps to replicate the issue (include links if applicable):

• Open Firefox with the default font-size settings (>20 pt)
• Reduce the width of the browser window (if needed; the width doesn't need to be small)
• Open a Wikipedia article in English Wikipedia while signed in
• Click on Edit source on top of the article (not a specific section)

What happens?:
The content does not fit into / adjust to the width like it used to.

What should have happened instead?:
The content should fit into / adjust to the width like it used to.

This works (or usually works?) when clicking on "edit source" at the top of a section rather than the whole article and it used to work until recently.

Software version (skip for WMF-hosted wikis like Wikipedia):

Other information (browser name/version, screenshots, etc.):
Firefox.
Screenshot below.
You can also see other new bugs in that screenshot. It's a mess. Namely: https://phabricator.wikimedia.org/T310893 and https://phabricator.wikimedia.org/T313817