The ending...

At first, I thought Joanna's change of heart was a bit too abrupt as well.

But when I watched the film again, I really watched her face towards the end during court.
You can see that she herself never realized that Ted even COULD be a good father, that Billy could be better off with him. She, like Ted at the very start of the film, had been completely wrapped up in herself.
She figured Ted just wanted Billy to be spiteful, or figured Ted couldn't be a parent 'for the long haul'.

But once she really sat down & thought about ALL the events, the entire sequence from her leaving all the way to the court's decision, she realized she herself might not be the best choice for sole custody.


I'd say this cloud is Cumulo Nimbus.
Didn't he discover America?
Penfold, shush.

I think the movie did a good job of establishing that Ted and Joanna are both basically reasonable people, capable of recognizing their own faults and reevaluating their impression of each other. Even though the movie is told from Ted's perspective, we're able to understand her point of view very well, how Ted was a workaholic who wasn't involved enough with the family and completely insensitive to what she was going through. In fact, if she'd fought for custody immediately instead of simply abandoning the family for a year, her case would have been a slam dunk: she was clearly better equipped to raise the kid at that point. What comes out in the court scenes is that she gradually realizes he has changed in the time she was away and has become a better and more attentive parent. It's not a sudden epiphany but something you can see happening as the case progresses. It only looks like a deus ex machina because it defies a common stereotype about how divorced couples behave.