when mirc reconnects to a server, and joins channels, it will automatically change focus from whatever other status or channel window you are in to the new channel.
problematically, this also happens when mirc is set to reconnect and rejoin with "keep channels open" turned on - with this option turned on, the fact that focus is moved to windows that already exist could be considered a bug - there is no "new" window being created - or a feature, but as a feature it interrupts whatever you are doing on IRC to call your attention to a disconnect that doesn't matter because you told mirc to rejoin open channels open anyway.
I would like to recommend changing the behaviour so that if the "keep channels open" option is selected, rejoining them on a reconnect does not interrupt use-flow.
- Pomax
