Scroll wheel test
Scroll up and down inside the box below. You'll see the direction of your last scroll, a running count split by direction, and the raw delta value your browser reported — useful for spotting a wheel that skips, sticks, or only registers one direction.
How the scroll wheel actually reports to your browser
Every scroll gesture — a mechanical wheel notch, a trackpad two-finger swipe, or a touch-screen drag on some devices — arrives at a web page as a wheel event. Its deltaY property carries both the direction and the magnitude: positive scrolls the page down, negative scrolls it up, and the size of the number reflects how far that single event represents (which itself depends on your OS and driver settings, not a fixed standard). This test reads that property directly, with the page's own scrolling switched off inside the test box so testing the wheel doesn't also move the page underneath it.
What to look for
- Both directions register. Scroll a few slow notches up, then a few slow notches down, and confirm both show up with the correct arrow and add to the correct counter.
- The count roughly matches your motion. A handful of slow, deliberate notches should produce a similar handful of events — a wheel that registers nothing for several notches in a row, then several at once, is behaving inconsistently.
- The delta value stays roughly consistent. For the same physical notch, on the same device, in the same browser, the number shouldn't swing wildly between one scroll and the next.
- No spontaneous events. If the counter climbs while your hand is nowhere near the mouse, that points to a genuinely stuck or oversensitive encoder rather than anything this page is doing.
Frequently asked questions
Why does my scroll count sometimes jump by more than one per notch?
Most mouse wheels have physical detents ("notches") but the wheel event doesn't have to fire exactly once per detent — the operating system's mouse driver decides how many wheel events, and what delta size, each physical notch produces, and some drivers or "smooth scroll" wheel modes send several small events per notch rather than one larger one. A single reading of more than one event isn't automatically a fault; a wildly inconsistent count for the same physical motion, repeated, is more telling.
My wheel only scrolls one direction. What does that mean?
That's a strong, specific symptom worth taking seriously — it usually points to a mechanical fault in the encoder that reads the wheel's rotation (a worn or dirty optical/mechanical encoder can lose signal in one rotational direction before the other), rather than anything software can fix. Try cleaning around the wheel first with compressed air, since dust genuinely does cause this on some mice, but a repeat failure after cleaning points to hardware wear.
Does this test also work for a trackpad's two-finger scroll?
Yes. Trackpads fire the same wheel event with the same deltaY sign convention when you scroll with two fingers, so direction and count both read correctly. The values will typically look smoother and more frequent than a mechanical wheel's discrete notches, which is expected and not a fault.
What's a normal delta value for one scroll notch?
There's no single number specified by any standard — it depends on your operating system, browser, and mouse driver settings (including any "scroll speed" or "lines per notch" setting you've configured). Common values cluster around plus or minus 100 for a single default notch in many browser/OS combinations, but this varies enough that comparing your own value to a friend's setup isn't meaningful. What is meaningful is whether the value stays consistent for you, notch after notch.