Re: [pointerevents] Add OS image dragging to non-normative reasons for pointercancel

The scenario I have in mind is draggable on-screen elements.  It ought to be reasonably simple to build basic drag and drop with `pointer{down, move, up, cancel}`.  Inside `down`, I can register a listener for `move`.  I can remove that listener on `up` or `cancel`.

If the [Observables proposal](https://github.com/tc39/proposal-observable/) lands, it would be effectively this:

```javascript
// (presuming single pointer for brevity)
const drag$ = down$.flatMap(move$.takeUntil(upOrCancel$).scan(distanceFromLastEvent));
```

An author ought to be able to call `dragStartEvent.preventDefault()` to prevent that drag stream from being interrupted by a `pointercancel`.  Otherwise, if the browser has taken control of the pointer stream, it should dispatch `pointercancel`.

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Received on Wednesday, 10 May 2017 19:47:26 UTC