Laptop display settings change when I plug the laptop in

Laptop display settings change when I plug the laptop in

my laptop behaves annoyingly when I'm using another display (laptop screen
+ another monitor, so in total two displays) with it. If I first connect
the other monitor, and then the power cable, Ubuntu mirrors the two
screens. Also, if I disconnect the second display, any applications whose
windows where on the second screen are "left there", i.e. if I click their
icon on the launcher they become active, but I cannot see them. They are
returned to the laptop screen only when I unplug my laptop.
My laptop is HP 635 and I'm using Ubuntu 12.04 LTS. I would like that the
display settings do not change in any way whether my laptop is plugged in
or not.