Just to document what exactly has happened:
It seems to be worst on the grey background – on a white background, it’s hardly noticeable. On a black background, it’s a thin white line.
I swear it’s gotten better, somehow – I didn’t take any photos when it had just happened (being too distraught and all), but there were patches of dead pixels and a distinct difference whenever something crossed the line. Now it seems like the dead pixels are gone, and it’s just a crack around which the backlight shows through some. My best guess is that the liquid from surrounding crystals has leaked or something, and it’s steadily going to get worse – the corner seems lighter than the rest of the screen.
Moving the screen causes waves of colour to cascade around the crack, and it may well be leaking, though not in any obvious way. It needs replacement, to be sure, but it’s far from unusable. I was considering an upgrade before, and this has just made that decision that little bit easier to make. Still, I wanted to give this one to mum, which would necessitate replacement – and all that aside, it still is a brilliant little system.