arc

The pure model behind angle3d and arc3d. An angle is a direction; an arc is a direction and a width. No DOM, no Babylon — so the part that is fiddliest to get right is the part a test can pin.

An arc is a CENTRE and a WIDTH, not a start and an end

Offered as start/end the two can be dragged into meaninglessness, and the interesting edit — "same width, point it there" — needs both moved together. Centre-and-width makes that edit one number, and makes the two hard questions one line each:

arcContains(arc, angle)          // |wrap(angle − centre)| ≤ width / 2
arcWithinArc(inner, outer)       // ...and the widths differ by enough

Written as intervals those need four cases and a wrap check, and every consumer would get one of them wrong. The interesting property is that neither form mentions where the arc starts, so wrapping past north is not a special case — it is unrepresentable.

Degrees, and two different wraps

Per this library's convention the surface is degrees. Two wraps exist and they are not interchangeable — see manipulator, which owns both:

wrapDegrees → (-180, 180], for a DELTA or a difference. Signed, because turning back five degrees is −5 and calling it 355 sends you the long way.
normaliseDegrees → [0, 360), for a STORED angle, which has no direction to preserve and reads better without minus signs.

A limit CATCHES, it does not jump

Drag past a stop and the handle stays at the stop. The alternative — snapping to whichever end is nearer in wrapped distance — means dragging a hair past one limit teleports the value to the other side of the dial, which is the worst thing a constrained control can do. So clampAngleToArc picks the nearer END, and the widget stops the gesture rather than restarting it elsewhere.

An envelope EXPLAINS, it does not merely clamp

The motivating case: "a game where you can place turrets on a ship and where it goes restricts the firing arc and the placement of the gun." A gun amidships on the port rail bears from roughly 10° to 190°, and no placement lets it fire through its own superstructure.

A control that silently snaps to a legal arc teaches nothing about why. So the envelope is a first-class value the widget can DRAW — the permitted sector and the blocked one — and clamping is what happens after you can see the reason.