Clear mousedown activation state whenever primary mousedown tracking is
reset. Also route local link drag cancellation through the drag-and-drop
end path when Escape, viewport leave, or mouseup outside the viewport
ends the drag.
These paths can otherwise leave drag input suppression active after the
user has ended or canceled the drag, or skip the required dragend event
for source cleanup.
Add an internals mouseLeave primitive and a focused text test. It covers
stale :active state, dragend dispatch for cancellations, viewport leave,
outside-viewport mouseup, Escape canceling, and later click delivery
after each canceled drag.
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