and you have long conversations with him at parties but he otherwise doesn't act interested

and he refuses to do anything work related outside of work

but about a year after the breakup, he sends you an email asking if you are going to an event at his office because he's going...

and it would be cool to hang out there...

did he just get hit upside the head with a cluestick or something?

Could just be a friendly invite to hang out.

probably keeping options open