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When writing a comment, you can type @so-and-so and a little popup will show you the mentionable people that match so-and-so. The new search options seem to use the same dropdown. However, for both, there doesn't seem to be a way to select them without using your mouse and clicking on the one you want.
Suggestion: Make the choices selectable by arrows keys and auto complete using the enter key, just like the tag dropdown when creating posts.