The window vision was an invention that Handy came up with in "Handy's Window Vision". It consists of a large powerful crystal that can transmit images of whatever is exposed to it to a bunch of smaller crystals, much like a television camera can transmit images unto dozens of television receivers.
Handy came up with this invention when he and Miner came across these crystals, amazed by seeing how an image can be transmitted from one crystal to another. Later on the village, Handy showed what these crystals could do to an amazed group of Smurfs, calling it "window vision". Soon every Smurf had a box with a receiving crystal in it that displayed whatever the other Smurfs were showing as "entertainment" in front of the transmitting crystal, including Papa Smurf, who was amazed when he saw Painter displaying a flip-book cartoon animation of Smurfs playing leapfrog. But Gargamel, finding one of the receiving crystals, would use it to find his way to the village, where he easily captured every Smurf except for Papa Smurf, Brainy, and Lazy since they were too busy watching window vision, and then took the transmitting crystal, seeing that it was the only powerful thing that could withstand the pressure of his Smurf Smasher. Unfortunately for him, the crystal proved too powerful as Gargamel was unable to stop the machine when his clothes got caught in it. It is presumed that the transmitting crystal was destroyed along with the machine.
Without the transmitting crystal, the receiving crystals were rendered useless. It was then that the Smurfs had bid farewell to window vision.