This fault seems to have suddenly developed overnight: in my web browser, my file browser, and in every program I've tested thus far, the program acts as if I am constantly pressing my left arrow key - which has all sorts of unwanted consequences. This still happens if I try another keyboard - or even with no keyboard attached.
xev -event keyboard registers no stray keyboard events.
My inxi -Fxzc0
I'm using i3 on an Architect install which has been (almost) fault free for over a year. The most recent change to the system was updating the kernel to the new 5.4 lts - I reverted to 5.3, but the fault persists.
EDIT: Well, I had no luck with any of the solutions offered here - thanks to y'all anyway - so I decided to bite the bullet and do what I've been putting off for a while: a full Arch install - the Archway.
So -thanks for all the fish, and goodbye! It's been fun and instructive, but I'm off...