When the currently shown page is the chat of a certain room,
prevent that room from moving around in the left pane due to new
messages/activity or unread/highlight counters change.
When the user switches to another page/room, the previously held lock is
released and that room completes all the moves it would have done if it
wasn't locked.
This makes navigating a room list with lots of activity easier, and
prevent annoyances like clicking on a room with unread messages and
having it immediatly fly down the list (possibly out of scroll view).
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