@2ck its actually really simple, its just a gird of tiny plates under the screen and their capcitance is constantly tested, when a change is detected it is seen as a touch.
your phone is likely creating capacitance but on the fringe of what the sensors will register. Because when your near the edge you arent surrounding the touch point with sensors on all sides like you might in the middle it has a lower sensitivity and your touch event is therefore falling out of range.
@2ck Oh I left out the most important part. There is also a constant voltage applied around the border of the screen. This is obviously needed to allow the capacitance to be detected (your body is interacting with the electric field to become the other plate of the capactor.
dope. thanks, @freemo
@freemo
That's a good point. Actually many phones reset their screens apparent capacitive threshold (the limit where it differentiate between an actual touch and just atmospheric charges and stuffs) when unlocked or locked. This same effect is also achieved by resetting and testing the screen through Android's built in test system.
@2ck