With the new Drupal8 Modal API I've been able to get a pretty powerful form going.
The form essentially does a POST to an API and displays some search results, which the user can filter. I want to paginate these results.
I'm having a lot of trouble wrapping my head around how this could be done, whether it's with custom ajax requests to increment some kind of counter that is used in the buildForm() function that does the API call, or something else. I've attempted to use Drupal's built in pagination but that doesn't seem like a feasible option as it doesn't even take route parameters.
A full example of the form can be seen at https://github.com/acbramley/embridge/blob/master/src/Form/EmbridgeSearchForm.php
Pagination seems to be broken in core through modals as well, when using a modal window to display (for example) the admin/content screen, pagination doesn't work.