Layout:
Use an absolute-fill view as the background with the sidebar on top of. This
greatly simplifies styling, as there is no need to calculate how large the
backdrop needs to be.
Animation:
Switch to a translateX transform animation. This serves 2 purposes: first,
there seems to be a bug somewhere in React Native 0.51-0.55 where the content
that is being animated starts to be clipped. Very weird! But more importantly,
translateX transmorm animations are supported by the native animation driver!
https://facebook.github.io/react-native/blog/2017/02/14/using-native-driver-for-animated.html8f5ebe5952/Libraries/Animated/src/NativeAnimatedHelper.js (L138-L176)
This makes the animation more performant and buttery smooth.
Some small cleanups are also included here.
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