How to use this test
- Make one slow turn inside the area. Read the deltaY sign in the log and select that expected sign.
- Reset. Slowly turn in one direction and compare the opposite-sign count with what you intended.
- Reset before testing the other direction. Inspect deltaX separately if your wheel supports horizontal input.
- Keep pace and connection constant for a second run. Save both reports; compare another mouse or application if unexpected events repeat.
Read the observations
Negative deltaY and positive deltaY are counted separately. These are signs of the browser event, not a promise about the direction that content will move under every system setting. Delta units are 0 for pixels, 1 for lines and 2 for pages. A wheel event is not necessarily one physical notch: high-resolution wheels, smooth scrolling and trackpads can emit multiple events or momentum after your gesture.
What this test cannot tell you
The page prevents scrolling only while your pointer is inside the test area. Event counts cannot establish encoder wear or measure the wheel mechanically. Natural scrolling settings, browser behavior and device drivers can alter the signal. An occasional reversal is a reason to repeat the comparison, not an automatic replacement verdict.
Need a closer comparison? Mouse scroll jumping: what to check and how to compare.
Questions about the test
Why do I get more events than notches?
The browser reports scroll deltas rather than mechanical notches. High-resolution input and smooth scrolling can produce several events per gesture.
Does unit 0 mean my mouse scrolls exactly that many screen pixels?
It describes the event unit. Actual content movement can still depend on the page and browser behavior.
Technical references
Method version 2.0 · Page updated 2026-10-20