Module wheel 

Wheel-delta normalization for shells: OS / DOM scroll deltas → the notch count Event::MouseWheel carries. Wheel-delta normalization: OS / DOM scroll deltas → agg-gui notches.

agg-gui’s Event::MouseWheel carries a notch count, not pixels: every consumer scales it itself (ScrollView multiplies by [PIXELS_PER_NOTCH], text areas and tree views do the same, and zoom consumers raise their per-notch factor to the power of the delta). Shells are therefore responsible for converting whatever their platform reports into notches, and this module is that conversion in one testable place — the native shells (LineDelta through, PixelDelta / 40) and the web shell (WheelEvent.deltaMode) had drifted apart, and a shim that forwarded raw pixels multiplied every scroll in the app by ~40-100×.

Precision deltas are forwarded fractionally

A macOS trackpad (and a smooth-scrolling mouse) reports pixel deltas of 1-10 px per event, i.e. a fraction of a notch. Those fractions are passed straight through: 4 px of finger travel is 0.1 notch, which a ScrollView turns back into 4 px, so precision scrolling stays 1:1 and continuous — no banking into 40 px jumps. A classic wheel still arrives as whole notches (one LineDelta line, or ~100 px per click in browsers that report wheels in pixels), so consumers that want a crisp per-click step get one; consumers must therefore scale by the delta’s magnitude rather than reading only its sign, or a trackpad would take a full step per 2-pixel event. Every consumer in agg-gui does (scroll containers multiply, zooms use step.powf(delta)).

An earlier revision banked sub-notch travel and emitted only whole notches. That made trackpads stutter in 40 px steps on the web while the native shell — which never banked — was smooth; the two shells now agree, via [to_notches] on both.

Structs

WheelNormalizer

Stateless per-event converter a shell keeps beside its wheel listener.

Enums

WheelDeltaMode

How a platform expressed its wheel delta. The discriminants match the DOM’s WheelEvent.deltaMode, so a web shell can convert with WheelDeltaMode::from_dom.

Constants

NOTCHES_PER_PAGE

Notches one “page” (DOM deltaMode 2) is worth.

PIXELS_PER_NOTCH

Pixels one wheel notch is worth. The scale ScrollView scrolls by, and the divisor the native shells apply to a trackpad’s pixel delta.

Functions

to_notches

Raw delta → notches, before any accumulation. Pure; see WheelNormalizer for the part that keeps state.