Jetpack: Comments: Threaded replies don't work with Jetpack 6.4

[dougaitken]

Steps to reproduce the issue

1. Using Jetpack 6.3.2 reply to a comment on the front-end. Comment will be threaded
2. Update to Jetpack 6.4
3. Reply to comment on the front-end

What I expected

When I press reply on a comment, it will thread the reply

What happened instead

Replying to a comment on the front-end on Jetpack 6.4, the reply is a new comment and not a reply
This can be seen in /wp-admin/edit-comments.php as the reply doesn't say "In reply to..."

Compare:
https://beta.doug.press/2018/04/hello-world/ on 6.4
https://delighted-toad.jurassic.ninja/2018/08/08/hello-world on 6.3.2

[zinigor]

Thanks for reporting this! This bug is apparently caused by #9681. Furthermore, I'm having some troubles sending comments with Firefox, which is fixed by rolling back the sandbox change introduced by that PR.

[StefMattana]

Dropping a ticket related issue just to keep track on that and cross-posting: 1333177-zen

[dougaitken]

Thanks for dropping the ticket link, Stef.

I've replied back to the customer to let them know we're fixing