This is a pretty slick indicator that your post is being fanned out to the #Fediverse ... might need to collapse that button with an edit/undo action though.
I can't believe the Threads team left out support for ALT tags in their latest #ReleaseNotes, but theirs is a brilliant implementation — if you leave them blank, Threads will automatically add a description.
"Soon, you’ll be able to follow and interact with people on other fediverse platforms, like Mastodon. They can also find you with your full username @username."