This prevents the user from going back. According to the React Navigation docs, it seems like it's only thing that should be necessary. But I found that without setting gestureEnabled the swipe-back would visibly complete, even though the screen would not technically dismiss, which led to an experience where I was looking at the prior screen but couldn't press anything or do anything.

I added headerLeft because I wanted to indicate to the user that going back was not possible at this time. I could probably have figured out a way to "disable" the button instead (ie. gray it out and prevent animation when pressed), but would be a bit more complicated, and I'm not sure it matters a ton.

We used to use $Shape for these types, but then we switched to $Partial when $Shape was deprecated. It looks like $Partial only supports undefined (and not null), but React Navigation definitely allows null here, and treats it as "no headerLeft" (as opposed to undefined, which is treated as "default headerLeft").