Facebook's official Messenger news blog announces product features such as end-to-end encryption, desktop apps, Rooms and Watch Together.
HN links it for official announcements, notably the rollout of default end-to-end encryption and the Watch Together feature.
This looks like the layout for the "Watch Together" feature: https://messengernews.fb.com/2020/09/14/feel-together-messenger-introduces-watch-together/
There's a bottom section that slides up and if a user clicks on one of the videos it will play for all participants. If anyone sees this and didn't intentionally click on one of those videos please file a bug!
Related: https://messengernews.fb.com/2023/12/06/launching-default-end-to-end-encryption-on-messenger/ (via https://news.ycombinator.com/item?id=38551993, but we merged that thread hither).