[SectionList][inverted] SectionList headers are sticky-at-the-top-footers if the list is inverted

• I have reviewed the documentation
• I have searched existing issues
• I am using the latest React Native version

Environment

Environment:
OS: macOS High Sierra 10.13.1
Node: 8.6.0
Yarn: 1.5.1
npm: 4.6.1
Watchman: 4.9.0
Xcode: Xcode 9.0.1 Build version 9A1004
Android Studio: Not Found

Packages: (wanted => installed)
react: 16.3.1 => 16.3.1
react-native: 0.55.3 => 0.55.3

Steps to Reproduce

Clone this repository and run it via react-native run-ios.
Observe the section separators are in wrong (randomly?) places.

Expected Behavior

The section list headers should be in the top of the section instead of the bottom like section footers do with an inverted list (but they aren't sticky).

Expected gif:

Actual Behavior

The section headers are footers instead of headers. They are sticky because of https://github.com/facebook/react-native/pull/17762 but still footers, not headers.

Sample code:

export default class App extends React.Component {
  render() {
    return (
      <View style={s.container}>
        <SectionList
          inverted
          sections={mock.sections}
          maxToRenderPerBatch={10}
          initialNumToRender={10}
          style={{ flex: 1 }}
          keyExtractor={(item) => item.messageId}
          renderSectionHeader={sectionProps => (
            <SectionHeader
              {...sectionProps}
            />
          )}
          renderItem={({ item }) => (
            <ListItem
              item={item}
            />
          )}
          ItemSeparatorComponent={ItemSeparator}
        />
        />
      </View>
    );
  }
}

Problem Gif:

Start by cloning the linked React Native reproduction and running it with react-native run-ios to observe the inverted SectionList behavior. Inspect the SectionList entry point and existing inverted-list handling, then compare the result with the expected and problem GIFs; done means headers remain at the top of their sections rather than behaving as footers.