fix: call a DX event after using an HTML draggable element (T1260277)

[chaosmirage]

An HTML draggable element triggers the pointercancel event after the dragstart event, as specified in the standards. However, this behavior was not handled by the events engine, leading to an endless event stream.

The user agent MUST fire a pointer event named pointercancel when it detects a scenario to suppress a pointer event stream.

https://www.w3.org/TR/pointerevents/#the-pointercancel-event

The user agent MUST suppress a pointer event stream when it detects that a pointer is unlikely to continue to produce events. Any of the following scenarios satisfy this condition (there MAY be additional scenarios):
[...]
As part of the drag operation initiation algorithm as defined in the drag and drop processing model [HTML], for the pointer that caused the drag operation.

https://www.w3.org/TR/pointerevents/#suppressing-a-pointer-event-stream

It seems this behavior was first introduced in the specification in 2018, as noted in the Pointer Events Level 2 Working Draft.

Safari still has a related issue.