Summary:
In iOS11, Apple added a new layout feature called "Safe Areas" (this blog post talks a bit about it: https://www.bignerdranch.com/blog/wwdc-2017-large-titles-and-safe-area-layout-guides/).
UIScrollView is one component that is affected by this change in Apple's API. When the `contentInsetAdjustmentBehavior` is set to `automatic`, for example, it will adjust the insets (and override any manually set insets) automatically based on whether or not there's a UINavigationBar, a UITabBar, a visible status bar, etc on the screen. Frustratingly, Apple decided to default to `Automatic` for this behavior, which will cause any apps that set contentInset/contentContainerStyle padding to have their values offset by, at the very least, the size of the status bar, when they compile their app for iOS 11. Here's more information about this behavior: https://developer.apple.com/documentation/uikit/uiscrollview/2902261-contentinsetadjustmentbehavior?language=objc
Mostly, this is a really straightforward change -- it simply adds a new iOS-only prop to ScrollView that allows setting `contentInsetAdjustmentBehavior`. But I did decide to default the behavior to `never`, so that it mimics the behavior we've seen in iOS < 11. I think it's good to keep something as crucial as scrollview content insets non-magical, and also keep it behaving similarly between platforms.
Closes https://github.com/facebook/react-native/pull/15023
Reviewed By: javache
Differential Revision: D5517552
Pulled By: hramos
fbshipit-source-id: c9ce4bf331b3d243228268d826fdd4dcee99981d
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