Not sure if a laptop screen is the same as a desktop monitor, but, with a desktop monitor, presumably, if one were to do that, one would then have a purple splodge at the top of their screen, which is usually fixed by degaussing. There is sometimes a degaussing tool in the monitor controls. Sometimes a screen will automatically degauss if you switch off and unplug the power supply for a longish period of time, overnight, perhaps?
Sorry if that's no help for a laptop.