XHR blocking broken in some webpages

[jinschoi]

Describe the issue

The XHR blocking fails on event dispatch with: "TypeError: Argument 1 ('event') to EventTarget.dispatchEvent must be an instance of Event".

Apparently, calling document.dispatchEvent with a CustomEvent created using "new CustomEvent(...)" throws that error, which can easily be tested using the console:

e = new CustomEvent("foo"); document.dispatchEvent(e);

Doing it using the old deprecated way: e = document.createEvent("CustomEvent"); e.initCustomEvent(...);

works just fine. In vapi-client.js, those two ways of creating CustomEvents are switched on legacyMode. If I just switch out "legacyMode" with "true" in line 600, it works properly for me.

One or more specific URLs where the issue occurs

I encountered this error on Flipster, a site to read magazines through a library subscription, but it can be seen here as well: http://www.javamagazine.mozaicreader.com/JanFeb2016#&pageSet=4&page=0&contentItem=0

Steps for anyone to reproduce the issue

1. Visit the example URL.
2. Note that it works with uBlock uninstalled (not just whitelisting the site).
3. Note that it fails with uBlock installed, even when the site is whitelisted.

Your settings

All default settings.

• MacOS version: 10.12.5
• Safari version: 10.1.1
• uBlock Origin version: HEAD

Your filter lists

Default, none, it doesn't matter.

[el1t]

The issue is that CustomEvent is being polyfilled by the website (e.g. the javamagazine one). I've got a workaround, though.