How the mouse scroll test works
The test area captures the scroll wheel, so the page does not move while your cursor is over it. For each wheel event the test shows:
- The direction of the last notch, and the count of notches up and down.
- Raw deltaY and deltaMode: what the browser actually sends, in pixels, lines or pages.
- Estimated notch size: the most frequent value, for example 100 px on Chrome for Windows or 3 lines on Firefox, depending on settings.
- Scroll speed in notches per second, now and peak over one second.
- A strip of the latest notches, up or down, with jumps in red.
A jump is counted when one notch goes the wrong way while the notches around it go the other way, all within 250 ms.
Method
- Scroll down slowly, 20 to 30 notches.
- Scroll up slowly, 20 to 30 notches.
- Scroll fast in both directions.
Slow scrolling matters most: that is when a dirty encoder misreads.
How to read the result
The verdict appears after 30 notches.
| Verdict | Condition | What to do |
|---|---|---|
| OK | 0 jumps | Nothing. The wheel is healthy. |
| Suspect | 1 or 2 jumps, under 2 % of notches | Clean the wheel and test again. |
| Faulty wheel | 3 jumps or more, or 2 % of notches or more | Clean the encoder. If it persists, replace it. |
How to clean a jumping scroll wheel
- Unplug the mouse.
- Blow compressed air into the gaps on both sides of the wheel.
- Spin the wheel quickly in both directions for 30 seconds.
- If needed, put a drop of isopropyl alcohol at the base of the wheel, spin it, and let it dry before plugging in.
Limits of a browser test
The number of lines per notch comes from your OS settings, so two computers can show different sizes for the same mouse. High resolution or free-spinning wheels (such as Logitech MagSpeed) and touchpads send many small deltas: the test flags this as smooth scrolling and the notch count is only an estimate. When you scroll very fast, the browser may merge several notches into one event.
Next step
A wheel that jumps often comes with a worn mouse. Check the main buttons with the double click test, or test every button at once with the mouse test.
Frequently asked questions
What is a scroll wheel jump?
It is a single notch that goes the wrong way in the middle of a continuous scroll. You scroll down and the page jumps up for a moment. In a game, it can switch to the wrong weapon.
Why does my scroll wheel jump?
The wheel uses a mechanical encoder. Dust, hair or wear in the encoder make it misread the direction, mostly when you scroll slowly. Compressed air, a drop of isopropyl alcohol and spinning the wheel often help. If not, the encoder needs replacing.
How fast can I scroll?
The test shows your current speed and your peak over one second, in notches per second. Fast flicks may be grouped by the browser into one event, which is counted as 2 or 3 notches.
Why does one notch show 100 pixels or 3 lines?
It depends on your browser and your system settings. Chrome on Windows usually sends 100 pixels per notch, Firefox often sends 3 lines. Windows scrolls 3 lines per notch by default.
Does it work with a free-spinning wheel or a touchpad?
It works, but these send many small deltas instead of clear notches. The test tells you so, and the notch count becomes approximate.
Updated October 26, 2026