Permission denied to access property eventPhase

> There was an attempt to access an object for which you have no permission.
—https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors/Property_access_denied

This is a frequent exception from Firefox browsers with extensions that
inject third-party scripts. The script dispatches an event that bubbles
up to the document listener which tries to access its eventPhase property.
This is not allowed because the event was generated from a third-party origin.

So attempt to read the property from the event, while catching errors,
to determine if we're allowed to use it. This seems like something the
browser should shield from the first-party site but here we are.

Author: dgraham

delegated-events.js

@@ -62,9 +62,19 @@ function defineCurrentTarget(event, getter) {
   });
 }
 
+function canDispatch(event) {
+  try {
+    event.eventPhase;
+    return true;
+  } catch (_) {
+    return false;
+  }
+}
+
 function dispatch(event) {
-  const events = event.eventPhase === 1 ? captureEvents : bubbleEvents;
+  if (!canDispatch(event)) return;
 
+  const events = event.eventPhase === 1 ? captureEvents : bubbleEvents;
   const selectors = events[event.type];
   if (!selectors) return;