HI folks - This is a long-standing defect, like from when we first switched to Discourse. I’m finally getting around to beefing about it.
Repro:
On any topic page, click the magnifying glass then begin by entering @ in the search box. A dropdown of posters’ usernames appears. Continue entering letters and the list auto-filters to just the names that match what you’ve entered. If you keep adding letters eventually the dropdown will filter to zero or just 1 name. And at any point in the successive filtering you can click a name and that auto-completes the @ entry in the search box and you can now continue entering other search commands or words.
All the above is normal and good and wonderful.
Now clear that search box with the little 'x" at far right, or start a fresh search on a different topic page.
Click the “in this topic” switch right below the search box. That moves an x in this topic entry into the search box. So far so good.
Now type an @. No response. No dropdown of usernames. Enter a couple letters that you know match somebody in the topic. Still no response; no dropdown of usernames.
Now click the little “x” in the x in this topic item to delete it. It disappears, but the @+couple letters you already entered is still sitting there with no filtered dropdown of matching names. Now add another letter or change the last one and the filter dropdown appears. Miraculous healing.
Bottom line:
The @ name search / filter feature is simply inoperative if the x in this topic is selected. There’s no obvious reason this should be so. I bet it’s a bug.