rent free
satisfied, dismissive
the one-line answer
Living in someone's head rent free means they cannot stop thinking about you without your effort.
Rent free means occupying mental space without paying for it. If someone is living in your head rent free, you think about them all the time even though they do not care about you. Used victoriously when someone cannot stop being affected by you..
mainstream
Your coworker might use this word.
01 / in the wild
hear it move.
observation
she said she's over him but he's living in her head rent free.
She claims to be over him but clearly still thinks about him constantly.