It's especially because of https://bugzilla.mozilla.org/show_bug.cgi?id=936729, we'll have different inline activities, and therefore I think we need a state in ThreadUI to just keep the fact that we're in an activity. It's really quick to just set a boolean and read it, and I think it makes sense.

We could also have this boolean in ActivityHandler with a getter. I can do this if you prefer. But still having a boolean here makes sense as the "ThreadUI" panel has a differant state. Tell me what you prefer :)