[higherkinded]

Trained human is likely capable of doing it exactly like the Puppeteer does minus the fact that the pointer may move instantly in case of the machine doing it. One wrong assumption covered, two more to go.

The second assumption made here is that the (as pointed out) fuzzing is a thing.

The third one is that you can't be sure of the input device. Joystick like the one seen on ThinkPad is uncommon but still used as the input device. Touchscreens are common but human-ish, though you will have to cover them too. The extreme case is that some X-savvy person may just move the pointer sometimes using the X's built-in capabilities, by dmenu and stuff, and even if that's highly unlikely, it still will fail.

There's no point in doing all the possible checks you may come up with: egde cases, no matter how impossible they look, still exist and break the consistency of the slider solution easily. Google captcha and Funcaptcha are nearly the best options you can readily get, freeing your hands and head of a huge deal of hassle you will face when dealing with this task.