JavaScript event handlers – PB Docs 2018

If you right-click an element and select Edit Events from the
pop-up menu, the JavaScript Event Handlers dialog box displays a
read-only set of event handlers for the element on the left:

This dialog displays the current JavaScript event handlers, if
any. You can add new event handlers or override the existing ones by
declaring them on the right side, or remove them by adding them with an
empty value.