Once a cause is understood, the solution can be found. My initial hypothesis was that it was a kind of RSI (asthenopia [see this]). But after further investigation and trial, that has changed to extreme dry-eye. RSI is on account of overuse, but the blink response is very frequent, and so overuse is unlikely. Indeed, underuse is the likely cause of some static load on one of the key eyelid muscles. This diagnosis has further been amended to inflammation with dry eye, after consultation with an eye specialist, being caused possibly by oral antihistimines and excessive computer work I took just before this problem arose.