Hardware diagnostic
Scroll Wheel Test
Roll your wheel inside the pad — every notch is logged with its direction, and any sudden reversal mid-scroll is flagged as a possible encoder skip.
Scroll here
Roll the wheel slowly one notch at a time in each direction, then fast. The page won't move while your cursor is inside this box.
Understanding scroll skipping
Inside most mice, the wheel turns a small rotary encoder whose metal contacts wear and collect debris over years of use. The symptom is unmistakable once you know it: you scroll down and the page stutters upward for a notch before continuing — or scrolling feels "slippery", moving further or less than the notches you felt. That stray opposite-direction event is exactly what this test flags: a reversal arriving within 120 ms of the previous event, far too fast to be an intentional change of direction.
To test properly, do three passes: slow single notches down and then up (each notch should log exactly one arrow, all the same direction), a fast continuous spin each way (watch the events-per-second stat and look for stray arrows in the log), and a few middle-click presses on the button test if your wheel doubles as a button. One flagged skip in hundreds of events can be a hand tremor; skips on every pass are a worn encoder. The fix ranges from a blast of compressed air, to firmware debounce options on some gaming mice, to soldering in a new encoder — a common and cheap repair.