If you have configured active corners on OSX and you are wondering why they don’t really work on your main screen I’ve got the reason why. If you look at your screen order from the screen settings dialog under system settings you can see if your screens are not of the same size, that the second screen is aligned on top right to your main screen. So the active let’s say top left corner of your main screen does not work because your mouse cursor is directly going to your 2nd screen. If you position the screens that way that the smaller screen starts a little below the main screens corner and above the main screens corner (see image) your active corners on the main screens side where the second screen is aligned to will work again. For example see my screenshot below, where my main screen is the big one on the right and the 2nd screen the small one on the left.