reuse styles (#3725)

packages/react-navigation/src/views/Header/Header.js

@@ -479,7 +479,7 @@ class Header extends React.PureComponent {
     return (
       <SafeAreaView forceInset={forceInset} style={containerStyles}>
         <View style={StyleSheet.absoluteFill}>{options.headerBackground}</View>
-        <View style={{ flex: 1 }}>{appBar}</View>
+        <View style={styles.flexOne}>{appBar}</View>
       </SafeAreaView>
     );
   }
@@ -575,6 +575,9 @@ const styles = StyleSheet.create({
     flexDirection: 'row',
     alignItems: 'center',
   },
+  flexOne: {
+    flex: 1,
+  },
 });
 
 export default withOrientation(Header);

packages/react-navigation/src/views/StackView/StackViewLayout.js

@@ -393,7 +393,7 @@ class StackViewLayout extends React.Component {
     if (headerMode === 'screen') {
       return (
         <View style={styles.container}>
-          <View style={{ flex: 1 }}>
+          <View style={styles.scenes}>
             <SceneView
               screenProps={screenProps}
               navigation={navigation}