On my laptop I have a hot-corner set to "Present Windows - All Desktops". When I connect to an external monitor I set my external monitor to the primary display. However, the hot-corner remains on my laptop when the external monitor is connected - this requires me to move the mouse very far from my main activity to reach the corner. I think it would make more sense for the hot-corner to move to the primary display by default, or have the ability to put the hot corner on all displays, or allow you to configure the hot corner exactly the way you want per-display.