When Batman finally stopped treating Harley’s self-destructive love for The Joker as a running gag.
Except that this is from Batman: The Animated Series. a.k.a. the incarnation of Batman that INVENTED Harley Quinn.
They never treated their relationship as a gag. They were brutal and in your face the entire series about how unhealthy it was.
Other Batman series may treat it as a gag but B:TAS never did.