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

• Edit this page with pending changes https://en.wikipedia.org/w/index.php?title=Diane_Abbott&action=edit

What happens?:

Overflow displays/

What should have happened instead?:
No overflow.

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

Other information (browser name/version, screenshots, etc.):
This is caused by setting width: 100% on a div with padding and perhaps some other CSS internal to the element. Divs by default by 100% width and browsers will kindly auto-adjust, so removing the 100% width is sufficient to fix this. ( https://gerrit.wikimedia.org/g/mediawiki/extensions/FlaggedRevs/+/a232bc70447e199e0d697bfcec6b8743e35aab6b/modules/ext.flaggedRevs.basic/default.css#37 )