I like how you're avoiding making MessageListCard more complex, but there seems to be a lot of dynamic HTML play here.  How about we just put the HTML for the status bar in the "card-message-list" DOM tree but mark it with the "collapsed" class to begin with?

Also, since this is very message list card specific, I think it might be better to put it closer to the MessageListCard and make the name more specific.  message-cards.js is getting pretty big, but maybe the right thing for us to do there is to split it in 2: message-list.js and message-reader.js ?