[ ] when you type a slash command, the autocomplete box will pop up. When you start typing an invalid slash command, it would be nice to have immediate feedback that the command is invalid instead of waiting until hitting enter (somewhat related to https://github.com/vector-im/riot-web/issues/4630)
[x] when typing a slash command, it would be nice if the autocomplete box stuck around after you typed the command name, since the autocomplete box shows the syntax of the command