Scroll Wheel Test
Hold the pointer over the pad and scroll in one direction. Wrong-way steps are flagged.
The page will not scroll while the pointer is on the pad.
Three wrong-way steps in a row are read as you changing direction on purpose, not as a fault. Pause for a second before scrolling the other way.
Raw deltaY values (newest first, reversals in red)
Reading the numbers
| Value | What it is | What a healthy wheel looks like |
|---|---|---|
| Down / Up | Wheel events received in each direction | Almost all in the direction you scrolled |
| Reversals | Single steps against the direction of the run | Zero, or very close to it, over a minute of scrolling |
| Unit (deltaMode) | 0 = pixels, 1 = lines, 2 = pages | Any; it depends on the browser and device |
| Raw deltaY | The size of each event as the browser reports it | Steady values on a notched wheel; many small values on a trackpad |
How this test works
The pad listens for the browser's wheel events while the pointer is over it, and stops the page from scrolling so you can scroll freely. Every event with a vertical value counts as one step, down or up.
A run starts on your first step, or on the first step after a pause of one second, and takes that step's direction. Any later step in the opposite direction is flagged as a reversal and shown in red in the raw list. If three opposite steps arrive in a row, the test reads that as you changing direction on purpose: it un-flags them and follows the new direction.
The raw deltaY values are shown because devices report scrolling very differently. A notched wheel usually sends one event per notch. Trackpads and free-spinning or high-resolution wheels send streams of small events, sometimes with momentum after you stop. Counting events rather than notches keeps the test honest across all of them.
Why a scroll wheel jumps the wrong way
Most mouse wheels turn a small rotary encoder. As the wheel turns, contacts or a light sensor inside the encoder produce two signals slightly out of step with each other, and the order in which they change tells the mouse which way the wheel moved.
When the encoder wears or collects dust, those signals get noisy. The mouse can then read the order wrong for a single step and report a scroll in the opposite direction: the page jumps up while you scroll down. That shows here as isolated red values in the raw list, scattered through an otherwise steady run.
Things people usually try, cheapest first: blow compressed air around the wheel while turning it, scroll hard through its full range a few dozen times, and check the mouse's software for a smooth-scrolling or wheel-acceleration setting that might be adding odd values. If reversals keep appearing on a notched wheel, the encoder is the likely part, and a warranty claim or a replacement encoder are the usual fixes.
Trackpads rarely produce true reversals; if you see them there, check whether two fingers drifted or a palm touched the pad. For a mouse whose buttons misbehave as well, run the double-click test.
Questions
How do you test a scroll wheel?
Put the pointer on the pad and scroll one way, slowly at first and then quickly. Every wheel event is counted as a step. A step in the opposite direction is flagged as a reversal.
Why does my scroll wheel scroll up when I scroll down?
The usual cause is a worn or dirty encoder, the part that reads the wheel turning. It reports an occasional step in the wrong direction, which this test flags.
Why does my trackpad show different numbers from a mouse?
Trackpads and smooth-scroll wheels send many small events, often with momentum after you lift off. Notched wheels send fewer, larger steps. The raw deltaY list shows exactly what your device sends.
What does deltaMode mean?
It is the unit the browser uses for the scroll values: 0 means pixels, 1 means lines and 2 means pages. Different browsers and devices use different units.