Scatter (Hearts of Heroes, #1) by Molly J. Bragg

When Deputy US Marshal Danielle 'Danny' Martin was told she'd gotten a promotion, she expected to be leading her own fugitive retrieval team. Instead, she got transferred to Pontian Florida of all places, and assigned to a Superhero support detail for Focus, a seemingly immortal superhero who is also one of the most famous lesbian icons on the planet.

Bad enough she's got to spend every day working with a woman she's had a crush on since she was five years old, but when she arrives at her new post, things start getting weird. It turns out that Focus asked for her by name, and it quickly becomes apparent that Focus wants to be more than just coworkers, or even friends.

After Focus has a violent reaction to Danny getting hurt in the line of duty, she starts looking into why the Superhero might be so fixated on her. She begins to suspect that seeing the future might be one of Focus' powers, but when a mission leaves her stranded thirty years in the past, right at the start of Focus's superhero career, everything becomes clear, except why the Focus in the past can barely seem to tolerate her presence.