Four ways to scroll sideways
Tilt wheel
Pushing the wheel left or right against a side switch. Common on office and productivity mice. On Windows this is frequently synthesised by the vendor driver, so uninstalling that software can remove the capability entirely.
Trackpad two-finger swipe
The most reliable source of genuine deltaX, including a momentum tail. Works out of the box on macOS, Windows precision touchpads and libinput.
Shift + wheel
A browser-level convenience. The event usually still arrives on deltaY and the browser redirects the scroll, so expect real sideways travel with vertical-looking events.
Free-spin thumb wheel
A second wheel under the thumb, found on some MX-class and trackball mice. Emits deltaX directly and often at a high event rate.
Reading the axis breakdown
After a run the report groups every wheel event your browser received into four buckets. They are mutually exclusive, so the four counts always add up to the total number of events.
- deltaX only
- Pure horizontal input. A non-zero count here proves your device can drive horizontal scrolling on its own.
- deltaY only
- Vertical events. Seeing these alongside sideways movement is the normal signature of Shift+wheel: the browser, not the device, changed the axis.
- Both axes
- Diagonal input, typical of a trackpad swipe that is not perfectly straight, and of free-spin thumb wheels during a direction change.
- Zero delta
- An event that reported no movement at all on either axis. A few are unremarkable; a steady stream of them during active scrolling is worth investigating with the mouse wheel test.
Fixing horizontal scrolling that stopped working
- Reinstall the vendor mouse software. On Windows, tilt-wheel input for most office mice comes from the manufacturer’s driver rather than from the HID stack. A Windows feature update that removes it will silently kill sideways scrolling.
- Check the app, not just the mouse. If this page sees deltaX events but your spreadsheet does not scroll, the input is arriving and the application is ignoring it.
- On a trackpad, confirm gestures are enabled. Two-finger scrolling can be switched off at the operating-system level, which disables both axes at once.
- Clean the tilt switches. They are ordinary micro-switches beside the wheel and they collect the same debris as the encoder.
For vertical problems — reverse scrolling, skipping, stutter — use the mouse wheel test, and see browser compatibility for how different engines report axes and delta units.
Horizontal scrolling questions
Hover the sideways track above and try each input in turn: tilt the wheel left and right if your mouse has a tilt wheel, swipe two fingers sideways on a trackpad, or hold Shift while turning an ordinary wheel. The axis breakdown after the run tells you which of them actually produced deltaX events and which produced nothing at all.
Most mice have no horizontal axis to report. Without a tilt wheel or a dedicated thumb wheel, the hardware simply has no way to signal sideways movement, so the browser receives nothing on deltaX. That is a capability limit rather than a fault — and it is why an empty horizontal count in this test is reported as “no horizontal input observed” rather than as a failure.
It scrolls horizontally, but usually without producing deltaX. In most browsers Shift+wheel keeps reporting the movement on deltaY and the browser itself redirects it to the horizontal axis, so this test will show vertical wheel events alongside real sideways travel. Firefox on some platforms does swap the axes at the event level. Either way the distance measurement is exact, because it reads how far the track really moved.
A page has to opt in: sideways scrolling only happens where an element is actually wider than its container. Beyond that, tilt-wheel input on Windows is often synthesised by the vendor driver rather than sent as a standard event, so removing the vendor software can silently disable it. Test here first — if the browser sees deltaX events, the input is reaching the system and the problem is in the app.
deltaX is the horizontal component of a browser WheelEvent, the sideways twin of deltaY. Like deltaY it is reported in pixels, lines or pages depending on deltaMode. This test counts events by which axes were non-zero — horizontal only, vertical only, both at once, or a true zero-delta event — so you can see exactly what your device emits.
Yes, and it is usually the most reliable way to produce genuine deltaX. A two-finger sideways swipe generates a continuous horizontal event stream on macOS, Windows precision touchpads and Linux with libinput, including the momentum tail after your fingers lift.