Bug 242627

According to my reading of the [drag and drop spec](https://html.spec.whatwg.org/multipage/dnd.html#dnd), as well as the behaviour of Chrome and Firefox, `dragenter` and `dragleave` events are supposed to set the related target event property (`event.relatedTarget`).

- `dragenter`: `event.relatedTarget` is the previous _current target element_ (where the drag is coming from)
- `dragleave`: `event.relatedTarget` is the new _current target element_ (where the drag has gone too)


When dragging into a window: `event.relatedTarget` should be `null` on `dragenter`

When dragging out of a window: `event.relatedTarget` should be `null` on `dragleave`

In Safari, for `dragenter` and `dragleave` events, the `event.relatedTarget` property is _always_ `null`. This is unhelpful. Doing this makes it painful to detect when a drag is entering / exiting the window. 


Reproduction case: 

- sandbox: https://codesandbox.io/s/event-relatedtarget-for-dragenter-and-dragleave-safari-bug-gpph01
- independent example: https://gpph01.csb.app/


As a work around you need to count `dragenter` and `dragleave` events to determine whether a drag is entering / leaving a window