I can't think of a single one, actually.

I seriously need to lighten up, watch fewer films noirs, and more comedies.
But part of the problem here is that the question is rather vague. What do you mean by a "happy ending," and what do you mean by "realistic"?
All comedies end happily, by their nature. As the old saying goes: comedies end with weddings, and tragedies end with funerals. In both cases, the social order that was disrupted by the story's events has been restored. Either the protagonist's legitimate ambitions have been satisfied, or their illegitimate ambitions have doomed them.
Shaun of the Dead is a good example. In the end, the hero not only survives the zombie apocalypse, he grows as a person, gets the girl, and still gets to hang out and play video games with his friend (though not as much as before)(and his friend is now a zombie).
Viewed from one angle, this is actually a "realistic" happy ending. Not everyone survives: a number of people die horribly, and Shaun's friend, as mentioned above, is zombified. Shaun's character growth and his reconciliation with Liz are believable.
But on the other hand--this a is a romantic comedy, with zombies. Is it even appropriate to talk about "realism" in relation to movies like this? Or moves like
Star Wars--surely one of the happiest endings in SF&F?
Furthermore: how happy does an ending have to be before it qualifies as a "happy ending"? Take, for example, the ending to
High Plains Drifter, mentioned above. This is certainly a
satisfying ending--justice has been done. But is it "happy"?
Take another Clint Eastwood movie--
Dirty Harry. In the end, Harry saves the busload of kids, and blows away Scorpio with the last round in his revolver. Why, then, does this victory feel bittersweet? Why does he take out his Inspector's star and throw it away? Is this a happy ending, or not?