Lonely Nights by @Hinn_Raven.ao3
After Stephanie Brown saves Martha and Thomas Wayne one night, instead of going back to her own universe, the Spoiler finds herself stuck in the wrong Gotham for a while longer, and gets to see some of the fallout of her choices. Sequel to "One Night" (previously recced) where Steph doesn't get home right away and has some more interactions with young Bruce. And the sequel, https://archiveofourown.org/works/23194855 Night Time, where grown up AU Bruce meets the Steph from his own universe.