In my previous post I tried to mention @sam, but the autocomplete list disappeared as soon as I pressed “s”. I tried doing the same for @codinghorror but the same thing happens, the list disappears as soon as I press “c”. I am pretty sure this was working few days back.
Also when I try to browse the initial list (which appears when I type “@”), the list disappears as soon as I press up and down key.
I tried doing same on try.discourse.org, I am getting same behavior there also.