This is basically a migration from mouse events to
[pointer events](
https://developer.mozilla.org/en-US/docs/Web/API/Pointer_events
).
The `PointerEvent` interface inherits all of the `MouseEvent`
properties, meaning that existing code can essentially be left
as-is. The only major change is making sure we only respond to the
"primary" pointer.
Known issues include:
* stylus hover is not detected
* touchscreens do not have a concept of hover, making it difficult
to e.g. resize areas
* no touch gesture support, e.g. "pinch-to-zoom"
