Requested feature overview description.
When you add a post to a pool, e621 should display the message "Added post to pool".
Like when you add a post to a set.

Why would it be useful?
Without this feedback, users will think the website is down or broken.

What part(s) of the site page(s) are affected?
Posts > Add to pool

Would be nice, I'm never sure if it actually added the post to the pool until I refresh to check.

edit: When it's successful it responds with location.reload(); which I presume at some point actually refreshed the page properly, but it's been 5 years since that code was touched and probably broke ages ago and nobody noticed. The error JS doesn't trigger either.