org.danielnixon.saferdom.impl.lib
The FocusEvent.
The FocusEvent.relatedTarget read-only property represents a secondary target for this event, which will depend of the event itself. As in some cases (like when tabbing in or out a page), this property may be set to null for security reasons.
MDN
https://w3c.github.io/uievents/#interface-focusevent