I sometimes accidentally change the behaviour of the view port navigation. I don't know how I do it, nor how to reset it.
The behaviour is different insofar that if I press the middle mouse button and move the mouse, the view does not rotate around a point that lies somewhere where I'm looking at but more like around a point in the "camera" of the view port.
It's like looking around in a first person shooter.
What mode is this and how do I change it back?