Bonjour Rosemary !
I understand your reasoning, and it makes sense, however, there is a nuance there in both English and French:
- When she wins, she will be happy. -> both actions are seen as simultaneous here
Quand elle gagnera, elle sera heureuse.
- When she's won, she'll be happy. -> here the action of winning precedes the happiness
Quand elle aura gagné, elle sera heureuse.
I hope that's helpful!
Bonne journée !