[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:

首先克隆链接的 React Native 复现项目，并使用 react-native run-ios 运行它，以观察反转的 SectionList 行为。检查 SectionList 的入口点和现有的反转列表处理，然后将结果与预期 GIF 和问题 GIF 进行比较；完成的标准是 headers 保持在各自 section 的顶部，而不是表现得像 footers。