Wiki pages are often too wide to read comfortably.
This extension adds "Toggle width" button that narrows the reading window.
See it in action:
Install TamperMonkey to your favorite web browser.
Once that's done, click here to install the script!
Alternatively, navigate to wikiwidth.user.js, click "Raw", then click "Install".
On every MediaWiki page, you'll find a new link called Toggle width under Tools.
Click that link to narrow the page, as in the demo above.
The script remembers your choice on a per-domain basis.
For developing, clone this repo and install wikiwidth.dev.user.js.
This will install the dev script, which you'll need to modify to point to wherever you cloned the repo.
Note that this works with Chrome but not all other browsers, since this requires local file system access.