[dave2000]

Read in the way a Facebook messenger message or a hangout message is read; you've got the "channel" open and the message is visible or is above the visible one. Clearly it's impossible to know whether a message has actually been read by a human but there Facebook/hangouts solution is almost perfect.

[josh2600]

It's not visibility, it's focus (at least on desktop). If you don't click a messenger box, it will display the message but not acknowledge it as read (on the desktop client, on mobile it works based on whether the message is displayed while the app is open).