Does Mattermost also have "uncollapsed" threads like Flowdock? Personally, I think hiding conversation in a group channel is terrible UX. I don't know why people are aping Slack's functionality when I hear almost universally that it sucks.
I'm not that familiar with Flowdeck so I don't know if this is what you mean, but "collapsed threads" in Mattermost is just a personal view setting that can be turned on/off according to your preferences.