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

• View a page with syntaxhighlighting such as https://en.wikipedia.org/wiki/Module:Documentation
• Observe the first code block

What happens?: The <pre> with its border stops at the content floating right. The surrounding <div> does not.

What should have happened instead?: The background should stop.

Software version (on Special:Version page; skip for WMF-hosted wikis like Wikipedia):

Other information (browser name/version, screenshots, etc.):

This is due to the various background CSS currently targeting div.mw-highlight. There's presently two blocks of it that will need to be removed, one of which is created by updateCSS.php. (Maybe one after T365774.)