The tunnel syndrome behaviour is something that kicks in shortly after booting. Blackness (not a shadow) spreads from the corners first, then from all the edges towards the centre, so that it creeps completely across the screen in about 20 mins., with the result that the machine is unusable for more than a few minutes at most. It is documented in posts here and there on discussion groups going back as long as a decade, so I doubt that the trouble is capacitors. Certainly, there is no known fix.

It it doesn't go any further than it has there, I'd not worry too much just yet.