Hover over a link to a page with a percent sign in the title e.g. https://en.wikipedia.org/wiki/99%25_Invisible
Instead of a preview of the page content, an error is shown
{"batchcomplete":true,"query":{"pages":[{"title":"99%25_Invisible","invalidreason":"The requested page title contains invalid characters: \"%25\".","invalid":true}]},"limits":{"images":5000,"categories":5000}}
The links in the action menu are also broken
