Certain input sources (such as touch) are "captured" when they
press an element. This means the pointer is always considered
"inside" the element by the browser, even when they visually are
not. This caused some issues on mobile browsers where touch and
stylus events could not connect table columns with each other.
Just to be safe, I've added the required `releasePointerCapture`
call everywhere `onPointerEnter` or `onPointerLeave` is used.
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"
