The first release: a menu bar app that lets your trackpad and your mouse scroll different ways, up and down and left and right.
Each device its own way
Four settings: the trackpad's up and down and left and right, and the mouse's. Each is Natural (the page follows your fingers, as on an iPhone) or Standard (scrolling down moves you down the page, as on Windows).
They mean the same whatever System Settings says, and keep meaning it when someone changes Natural scrolling there.
A Magic Mouse is a mouse. Newel tells devices apart by which one sent each scroll, not by guessing from how it moves; a scroll another app sends, such as Logi Options+, counts as the mouse's.
Until you choose, Newel changes nothing: scrolling stays the way System Settings has it.
Or let it find out
The welcome asks you to scroll down, then right, over a page that always moves the way it asks, and sets each device from which way your hand went. Scroll the other way to change it.
The same test is in Settings › Scrolling, with the four settings right under it.
Scrolls that behave
A flick keeps its direction to the end of the coast. A changed setting, a pause or a resume applies from the next scroll, never halfway through one.
Shift with a mouse wheel scrolls sideways and follows the mouse's left and right. With ⌃, ⌥ or ⌘ held too it's a shortcut (Colonnade's ⌃⌥⇧ + wheel), and keeps up and down.
Swiping back and forward in Safari, Chrome and Firefox still works with a trackpad's left and right turned around, and Safari scrolls without a bounce.
Newel's tap goes first, before any other app's, so Colonnade's ⌃⌥ scroll and the like see the direction you chose.
Safe to leave running
If another app turns scrolling back after Newel (Scroll Reverser, say), the menu and Settings say so and name the apps that could be doing it.
Take Accessibility away, switch to another user, or quit in the middle of a scroll, and scrolling is simply the system's again; nothing sticks.
If macOS turns Newel's tap off, Newel turns it back on at the next scroll. If it can't keep up at all, it stops and says so, and scrolling is the system's.
Each scroll takes Newel about 15 µs. At rest it uses no CPU.
The rest
Pause from the menu bar: scrolling follows System Settings until you resume.
newel in Terminal: state, pause, resume, set, export and import, diagnostics, and help for the rest. newel://pause and newel://resume work as links, for Shortcuts and Raycast.
It asks for Accessibility and nothing else, and changes only which way scrolling goes: no smoothing, no acceleration, no remapping. Nothing about you leaves your Mac; Diagnostics (Settings › About) has counts only, never an app's name or where the pointer was.
