Scroll wheel test - check for jumping and stray ticks

The fault worth finding

If your page jumps back up while you are scrolling down, or a value flicks the wrong way when you are adjusting it, the cause is almost always the rotary encoder inside the wheel. As it wears, or picks up dust, it emits the occasional tick in the wrong direction.

It is to a scroll wheel what chatter is to a mouse button, and it is found the same way: by looking for something a hand cannot do.

How this tells a fault from you changing your mind

You can of course reverse direction deliberately, so a reversal on its own means nothing. What you cannot do is reverse within a few tens of milliseconds and then immediately carry on in the original direction. So a reversal is only counted as a fault when it is both inside that window and followed by a return to the previous direction - a single stray tick embedded in a continuous scroll. Deliberate changes of direction are never counted.

How to test

1. Scroll inside the box at your normal pace - do not scroll unusually fast.
2. Do at least thirty events in each direction. The fault is intermittent, so a handful proves nothing.
3. Watch the stray tick count and the log. Any row marked Stray tick is one event the wheel reported in the wrong direction mid-scroll.
4. If your mouse has a tilt wheel, push it sideways to check horizontal scrolling too.

If stray ticks are found

1. Clean it first. Dust in the encoder is the most common cause and by far the easiest fix. Compressed air around the wheel, spinning it as you go, resolves a good share of cases.
2. Check the warranty. A wheel that jumps is a recognised fault and gets replaced.
3. Replace the encoder. Out of warranty they are inexpensive and the repair is well documented for popular mice.

Reading the other numbers

Reported in shows the unit your browser uses - pixels, lines or pages. This varies by browser and operating system: Firefox on Windows commonly reports lines where Chrome reports pixels, so a delta of 3 in one and 100 in the other can be the same physical notch. Counting notches without accounting for that is why some wheel tests give wildly different numbers between browsers on one machine.

Step size is the typical delta per event. A notched wheel produces a consistent value; a wide spread means smooth scrolling.

Trackpads and free-spinning wheels

A trackpad, and a wheel in free-spin mode, produce a stream of small smoothly varying deltas rather than discrete notches, and their momentum scrolling naturally decays through zero. Judging those by a notched wheel's rules would report a fault on perfectly healthy hardware, so smooth input is detected and the stray-tick check is suppressed for it. If the page says smooth scrolling was detected, that is why.

Related checks

Our mouse test checks the buttons and movement, and the double click test finds the other common mouse fault - a switch that sends two clicks for one press.