Steps to reproduce:
* Reply to a comment in a Structured Discussions thread, using the visual mode
* Add a link to `[[Topic:Ush5xp8bvwk69y2l]]`.
* After you've made it, click that link to open it in another browser tab (e.g., to make sure that you linked the correct discussion).
* See the error message from your web browser about the website `Topic:Ush5xp8bvwk69y2l` not existing. The link ought to be `https://www.mediawiki.org/wiki/Topic:Ush5xp8bvwk69y2l` instead of just `Topic:Ush5xp8bvwk69y2l`.
It looks correct in the editing mode, and the link works correctly after the comment is saved: