On my other PC, the mouse pointer is bouncing around the screen. I've been away for a few days and it wasn't doing this before.
The pointer is hopping from spot to spot in what could be a chaotic motion i.e. I can't see a pattern. It hops several times per second and each hop is usually no more than a couple of inches across my (26") screen, usually in a different direction, but over time it's migrating around the whole screen. Both button clicks are also happening: whilst I was pondering on it, it managed to change to the High Contrast display theme.
I've never seen anything quit like it. The nearest thing was way back when I was playing with plotting mathematically chaotic equations.
I am not moving the mouse.
Although the symptoms appear similar, this is not an exact duplicate of "Why is my mouse pointer moving on its own?" - in that question, the pointer remains on the far right of the screen unless the mouse is moved. In this question, the pointer is flickering around across the whole screen but responds partially to mouse movements, by moving the centre of the flickering around the screen. The only similarity between these questions is that the pointer is not behaving as it should - the actual behaviours are entirely different.