Update: Windows seems to have resolved the problem on its side without any updates that I could see in Update History.
A couple of days after first posting here I thought I had stumbled on an even easier workaround when I had a browser open while closing SL and there was no bad-trip psychedelic mess on screen.
Since then I've closed SL several times with nothing else running and no graphics problem. How it resolved itself, I have no idea and I don't care.
And no, I did not do the logical thing and download the Rainbow RC viewer because I am, indeed, just that lazy. 😃