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Document keeps track wheter a certain event listener is available in the DOM, we only dispatch certain events if document()->hasListenerType(..) returns true.
In Node::addEventListener, we're adding the listener type, but in Node::removeEventListener we're not notifying the Document that something changed.
We need to switch to a counting based tracking scheme, counting how many event listeners are registered for each event type.
This bug can introduce performance problems, when adding a mutation event listener once for ie. <body> and then removing it. We will never stop dispatching mutation events.
Does this happen on real web sites? Counting listeners would add a toll on every web site, while benefits seem unlikely to materialize.