Description
Add a menu to replace the current "Add to set" popup window.
This menu could be similar to reddit's "Add to custom feed" menu, as in it displays all sets as a list instead of as a dropdown, and allows for checking/unchecking multiple sets before submitting.

Why would it be useful?
It would make changing adding or removing posts to/from sets a lot more convenient, as multiple sets could be affected at once.
In addition, it would provide a more comprehensive way of removing posts from sets, than to go through the post listing page.

What parts of the page would be affected?
Individual post page, possibly post listing page.

I wonder how this will be for users that have hundreds of sets though

faucet said:
I wonder how this will be for users that have hundreds of sets though

A popup window could include pagination. Perhaps even a name search. Which - in my opinion - would be way more manageable than a scrollable dropdown.

I want the last used set to be preselected, even after cookies are cleared. It seemed to be like that after the major site update a couple of years ago, but now it's just the first in the list again.