Event rule

The rule

An event listener operates in the context of the code that dispatches the event, if that context is not the root context. Else, the listener operates in the context of the addEventListener() use, or of the assignment to the on... property.

Examine the rule

Add a listener in context A.
Set onclick in context B.
Click the large button. The browser dispatches the event, so each handler gets its registration context.
Dispatch a click from context C. Code in context C dispatches the event, so each handler gets C.

Add the handlers, then click the button. Each click adds a line here.

A green line is a click in which the handler got the context of the rule. A red line is a click in which it did not.