Interesting, saw this come up a few times when I worked at Skype a very long time ago and never entirely understood it.
If Gitter isn't open and in focus you don't appear green in the room. So if you just switch tabs or applications - why is there a need to not have something selected?
@mydigitalself Mainly because sometimes I don't have enough time to read all chats and as a consequence of this I don't want accidentally mark some messages as read while switching between tabs in browser (I'm using the web version of gitter, pinned as a first tab in my browser).
Btw Skype solves this by a "move bookmark here" feature.