How the scroll wheel test works
Put the pointer on the pad and roll the wheel. The page does not move, because the pad takes the scrolling for itself. Every step is counted as up, down or sideways, the arrow shows the direction of the last one, and the list at the bottom records them in order. Pressing the wheel down counts as a wheel button press.
Roll steadily in one direction for at least 15 steps, then the other way. With 15 steps each way the test gives a verdict.
What a reversal is
The fault people describe as “my scroll wheel jumps” is the wheel reporting the wrong direction for a step: you roll down, and somewhere in the middle the page hops up. The test counts a reversal whenever the direction flips within 120 ms of the previous step, which is too quick to be you changing your mind.
One or two reversals can come from your finger rocking the wheel back as it lifts. Reversals that keep appearing while you roll steadily in one direction point to the encoder, the part that reads the wheel’s rotation. In most mice it is mechanical, and it misreads when it is dirty or worn.
Line steps and pixel steps
The list labels each step as a line step or a pixel step, with a size. That is how the browser chose to describe the movement, and it depends on your browser and operating system. It says nothing about the health of the wheel, so do not compare the sizes between computers.
Free-spinning wheels and trackpads
A wheel with a free-spin mode, a touch-sensitive wheel and a laptop trackpad all send a stream of small steps rather than one step per click of the wheel, and some keep sending them after you have let go. Expect high counts. Reversals should still be zero when you scroll in one direction.
If the wheel is misbehaving
- Blow it out. Aim compressed air into the gap beside the wheel while turning it. Dust and fluff on the encoder are the commonest cause, and this often cures it.
- Check the software. Smooth scrolling or acceleration options in the mouse’s own software can make a healthy wheel feel erratic. Turn them off and test again.
- Try another computer. If the reversals follow the mouse, it is the mouse.
- Warranty or repair. A worn encoder does not recover. It can be replaced, which needs soldering, or the mouse can go back under warranty.
To check the buttons as well, use the mouse test. If the wheel button double clicks, the double click test times it.