Question 1

What does the scroll wheel test catch?

Accepted Answer

It hooks the wheel and wheel-tilt events the browser emits as you scroll. Each tick is logged with its deltaY (vertical) and deltaX (horizontal) magnitude. Skipped detents, ghost ticks in the wrong direction, and dead zones at the start of a roll all show up immediately because every event is rendered as a coloured bar on the timeline. If you roll cleanly through 20 detents and only 18 register, the encoder under the wheel is failing — a common wear point on mice past 3–4 years of heavy use.

Question 2

Why does my wheel scroll backwards occasionally?

Accepted Answer

Inside the scroll wheel is an optical or mechanical encoder that reads two offset signals; the direction is inferred from which signal changes first. When the encoder wears, signal edges get noisy and the controller occasionally misreads the order, producing a single tick in the wrong direction. One stray tick per hundred is borderline tolerable. A pattern of reverse ticks at the start or end of every scroll is encoder failure and almost always means a new wheel module or a new mouse.

Question 3

Does horizontal tilt work in every browser?

Accepted Answer

Tilt fires a wheel event with a non-zero deltaX. All modern desktop browsers expose it; mobile browsers do not because phones lack a physical wheel. Some mice tilt the wheel mechanically (Logitech MX series, most office mice) and some emulate tilt with a thumbwheel (Razer Basilisk). Both surface as deltaX in JavaScript. If your tilt feels stiff or registers only one direction, the side switches under the wheel — not the encoder — are at fault and they are replaceable individually.

Question 4

What is the difference between line, page, and pixel scroll modes?

Accepted Answer

WheelEvent.deltaMode reports 0 for pixel scroll, 1 for line scroll, and 2 for page scroll. Free-spinning wheels (Logitech MX Master, Apple Magic Mouse) emit pixel mode with small deltas; click-detent wheels emit line mode with delta values of 100 or 120 per detent depending on the OS. This tool normalises both into a tick count so a single detent always reads as one tick. The raw deltaY is shown if you want to see the unscaled value.

Question 5

My scroll feels too fast or too slow — is that the mouse?

Accepted Answer

Usually no. Scroll sensitivity is the product of three multipliers: the mouse's reported delta, the OS scroll speed setting, and the application's own scroll rate. Windows defaults to 3 lines per detent; macOS defaults to smooth pixel scrolling. Chrome adds its own per-tab smoothing. The tick count this tool shows is the raw delta event count and is independent of perceived speed. If the count is correct but scrolling feels off, adjust the OS slider, not the mouse.

Question 6

Can I test middle-click here?

Accepted Answer

Middle-click is a button press on the wheel and is captured by /mouse/buttons, not by this tool. This page only listens for scroll-axis events. If your wheel scrolls cleanly but middle-click fails, the tact switch under the wheel — separate from the encoder — has died. Many premium mice now use optical wheel-click switches that outlast the encoder, so on those the middle button is the last thing to fail.

Scroll wheel.