Weirdest thing this morning. My mouse stopped working right. I could move the cursor but not click the mouse. So I swapped it out for another mouse. Same problem. So I rebooted. Same problem. I then switched to a wireless mouse and then a Bluetooth one. Same problem across the board.
I won’t make you sit through all the problem solving that went on: same issue meant that this was not a mechanical error, and not tied to, for example, specific wires, or bulging batteries or whatever. The tl;dr is this: I had taken out a magic trackpad a few hours earlier, intending to use it (but never got around to it), and left it on a counter and a child had put something on top of it.
The magic trackpad had not only powered on but was continuously, due to the weight, issuing some sort of mouse press because of the weight of the stuff dumped on top of it. Once I took the weight off, everything started working again back at my computer.
Diagnostically: the cursor moves, any right-button works, any scroll wheel works, but not the left-button. Solution: hunt around for a wireless pointing device that might be interfering. If you have Screen Sharing enabled, you can disable Bluetooth and see if that resolves the problem.
I took the batteries out of the trackpad, and put it away gently.
I’m leaving this blog post in case it ever helps anyone else out on this very weird issue. The advice out there on the web all assumes a mechanical issue either with a built-in trackpad, with a pointing device, or a system issue. This was such a sideways situation that surely I can’t be the only person it will happen to but it probably most everyone will never be affected.