Drag (a gesture with a lifetime)

How it works

Each drag is a process: spawned on pointerdown, yielding offsets, disposed on pointerup. The gesture's whole footprint — its subscription included — ends when it does; nothing about it is global.

const gesture = spawn(async function* (self: Self<PointerEvent>) {
  for await (const move of self)
    yield { dx: move.clientX - down.clientX, dy: move.clientY - down.clientY }
}, undefined)

// pointermove: gesture.send(e)
// pointerup:   gesture[Symbol.dispose]()   — the gesture is over