UserAgent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/70.0.3538.110 Safari/537.36

Steps to reproduce the problem:
1. <a href="http://haveibeenpwned.com" onclick="window.open('http://twitter.com')">popunder?</a> (or use a quick demo page: https://bruteforcemovable.com/popunder.html)
2. Click on the link

What is the expected behavior?
Browser should have blocked one of the page transitions. (Probably the onclick one since its destination is not shown on link hover)

What went wrong?
Chrome (And Firefox, and MS Edge) open both Twitter in a new tab (which gets focus) and haveibeenpwned.com in the old tab (which is not focused).

Did this work before? N/A 

Chrome version: 70.0.3538.110  Channel: stable
OS Version: 10.0
Flash Version: 

I'm not quite sure why this works. I thought only 1 page transition per user interaction is allowed. 

Since this is such a basic example im probably missing something, if that is the case please enlighten me.