Mouse Scroll Test
Scroll up and down in the box below. The indicator moves with your scroll direction, and every raw wheel event is logged so you can see step size and spot skips for yourself.
Scroll with the wheel or two-finger swipe. The dot moves up when you scroll up, down when you scroll down.
What this actually tests
Scrolling reaches the browser as wheel events carrying a deltaY (vertical) and deltaX (horizontal) value per notch or swipe. A healthy wheel produces roughly consistent deltas for a consistent scroll motion; a wheel with a worn encoder tends to produce erratic values, skipped steps, or occasional large unexpected jumps — all visible in the raw log above.
If scrolling feels wrong
- Check the deltaY sign: scrolling up should be consistently negative (or consistently positive — direction convention varies by OS, but it should be consistent one way).
- Look for large, isolated jumps in the log — that's the classic symptom of a wheel encoder starting to fail.
- Clean around the wheel with compressed air; dust is a common cause of skipping.
- Check your OS mouse settings for "reverse scroll direction," which is sometimes toggled by accident.
Scroll wheel not responding at all?
Read the fix guideFAQ
Why does my scroll wheel skip every few notches?
Skipped notches almost always trace back to the mechanical or optical encoder inside the wheel losing contact briefly, usually from dust buildup or wear after a few years of use. Compressed air around the wheel clears the majority of cases; if skipping continues after cleaning, the encoder itself has likely worn out.
What's a normal deltaY value per scroll notch?
It varies by OS, browser, and mouse — commonly somewhere around 100-120 per notch on Windows or a smaller, smoother value on trackpads and precision mice — so there's no single "correct" number. What matters is consistency: the same physical scroll motion should produce roughly the same deltaY each time, not the exact figure itself.
Can a driver update fix erratic scrolling?
Sometimes — a corrupted or outdated mouse driver can misreport wheel events even when the hardware itself is fine, so it's worth ruling out before opening the mouse. If a fresh driver install doesn't change the erratic pattern in the log above, the encoder is the more likely cause.