If I paste in this link:
https://www.youtube.com/watch?v=Nxc4AKz_lhQ&list=PLh1XHNREyQEPd-B4Dnlhg1ly7CHbbdW0f
It will be rendered thusly:
In other words, the link has been cut down to this:
https://www.youtube.com/watch?v=Nxc4AKz_lhQ
The playlist suffix is no longer present. Either these types of links should not be oneboxed, or they should be appropriately oneboxed as a playlist, as demonstrated by WordPress:
@techAPJ can you have a look at this?
Just a reminder, is this on the list @techapj?
Yes this was on my list and I looked into it few days back, but there was no practical way to do it using lazyYT.
So what I did now is switch to traditional iframe embed when user tries to embed playlist. It will now be rendered as playlist:
https://github.com/discourse/discourse/commit/076ed66e690bb9a8899103f75ce47560594d0bf0
Let’s see if this playlist gets oneboxed as expected 
