It can be moved if you're not scrolled to the bottom of of the window
or the window doesn't have focus when a new message comes in. Other than
that, it marches up the window until you close and reopen the
conversation, or send a message.
Note that we do NOT mark messages as read if they come in when you are
scrolled up. But we do mark the entire conversation as read if you
switch away from the app and back.
FREEBIE
* Remove increment behavior
* Dismiss when new messages arrive but the window is focused
* Update the indicator when window becomes focused.
// FREEBIE
This is to ensure that when there are a lot of unread messages, the user
is given the chance to see all of them by being scrolled to the oldest
new message.
When a new message comes in, the indicator will be incremented.
When the user sends a message or switches away from the conversation,
the last seen indicator will be removed.
FREEBIE
