ScrollView with flex 1 makes it un-scrollable

Try using flexGrow: 1 instead of flex: 1 in scrollView content container style as follows.

<ScrollView contentContainerStyle={{ flexGrow: 1, borderColor: 'green', borderWidth: 5 }}>
  <View style={styles.box1} />
  <View style={styles.box2} />
  <View style={styles.box1} />
</ScrollView>

https://snack.expo.io/HkkEVoh6Z