Matching with existing mentions and whitespaces (#11400)

In this test, a couple of feed items were provided for the @ marker. The most relevant, though, is @marry that matches against three different entries.

The point of this test is to make sure the UI will not show up when the selection is after an existing mention even if there's a white space between them and it matches against one of the items in the feed ("Marry Foo", "Marry Bar", or "Marry Baz").

  1. Put the caret at the end of the mention. The UI should not show up.
  2. Type a whitespace after the mention. The UI should not show up.
  3. Create a new line and type @marry. The UI should pop up with three suggestions.
    1. After accepting the mention, the UI should disappear and stay out of sight.

@marry