Scroll test
Your wheel jumps backwards sometimes. This proves it.
Scroll inside this box
Scroll steadily one way, then the other
How to test a scroll wheel
A scroll wheel has an encoder inside it, and when that encoder gets dusty or wears down it starts lying. The classic symptom is a page that jumps back up while you are scrolling down — brief enough that you blame yourself. This test records every notch: how large each step is, and whether the direction ever flips mid-scroll. A healthy wheel produces one or two consistent step sizes and never reverses. It also shows which unit your browser is reporting, which quietly tells you whether a driver is intercepting the wheel.
- 1Put the cursor inside the box and scroll steadily downwards for a few seconds.
- 2Then scroll steadily upwards for a similar stretch.
- 3Keep going until you have at least twenty notches recorded.
- 4Check the reversal count — on a healthy wheel scrolled in one direction it should stay at zero.
FAQ
What does a reversal mean?
The wheel reported a notch in the opposite direction to the one before it. When you deliberately change direction that is expected. When it happens while you are scrolling steadily one way, the encoder is misreading.
Why are there many different step sizes?
A notched wheel should produce a small set of repeating values. Many different sizes usually means a touchpad or a smooth-scrolling wheel, both of which send pixel-level deltas — that is normal for those devices.
What is the unit line telling me?
Browsers report scrolling in pixels, lines or pages. A notched wheel usually reports lines and a touchpad reports pixels. Seeing both in one session means something is changing the mode as you scroll, typically driver software.
My wheel skips but shows no reversals.
Try scrolling more slowly. Fast flicks can outrun the encoder in a way that looks like skipping but is really the sensor sampling rate, not a defect.