Add tests describing current sticky header realization behavior

See react-native-community/discussions-and-proposals#335 for extra context.

A VirtualizedList may have sticky headers, forwarded on to ScrollView. These sticky headers are exempt from virtualization once realized for the first time. This change documents the behavior of keeping sticky header cells realized after scrolling away.

This scenario performs the same behavior as creating an internal "realization window" for sticky headers with a single cell window size. Generalizing the concept of realization windows should be shaped to support the existing sticky header scenario.

Libraries/Lists/__tests__/VirtualizedList-test.js

@@ -504,4 +504,163 @@ describe('VirtualizedList', () => {
       'scrollToIndex out of range: requested index 3 is out of 0 to 2',
     );
   });
+
+  it('forwards correct stickyHeaderIndices when all in initial render window', () => {
+    const items = Array(10)
+      .fill()
+      .map((_, i) => (i % 3 === 0 ? {key: i, sticky: true} : {key: i}));
+    const stickyIndices = items
+      .filter(item => item.sticky)
+      .map(item => item.key);
+
+    const ITEM_HEIGHT = 10;
+
+    const component = ReactTestRenderer.create(
+      <VirtualizedList
+        data={items}
+        stickyHeaderIndices={stickyIndices}
+        initialNumToRender={10}
+        renderItem={({item}) => <item value={item.key} sticky={item.sticky} />}
+        getItem={(data, index) => data[index]}
+        getItemCount={data => data.length}
+        getItemLayout={(_, index) => ({
+          length: ITEM_HEIGHT,
+          offset: ITEM_HEIGHT * index,
+          index,
+        })}
+      />,
+    );
+
+    expect(component).toMatchSnapshot();
+  });
+
+  it('forwards correct stickyHeaderIndices when partially in initial render window', () => {
+    const items = Array(10)
+      .fill()
+      .map((_, i) => (i % 3 === 0 ? {key: i, sticky: true} : {key: i}));
+    const stickyIndices = items
+      .filter(item => item.sticky)
+      .map(item => item.key);
+
+    const ITEM_HEIGHT = 10;
+
+    const component = ReactTestRenderer.create(
+      <VirtualizedList
+        data={items}
+        stickyHeaderIndices={stickyIndices}
+        initialNumToRender={5}
+        renderItem={({item}) => <item value={item.key} sticky={item.sticky} />}
+        getItem={(data, index) => data[index]}
+        getItemCount={data => data.length}
+        getItemLayout={(_, index) => ({
+          length: ITEM_HEIGHT,
+          offset: ITEM_HEIGHT * index,
+          index,
+        })}
+      />,
+    );
+
+    expect(component).toMatchSnapshot();
+  });
+
+  it('realizes sticky headers in viewport on batched render', () => {
+    const items = Array(10)
+      .fill()
+      .map((_, i) => (i % 3 === 0 ? {key: i, sticky: true} : {key: i}));
+    const stickyIndices = items
+      .filter(item => item.sticky)
+      .map(item => item.key);
+
+    const ITEM_HEIGHT = 10;
+
+    const virtualizedListProps = {
+      data: items,
+      stickyHeaderIndices: stickyIndices,
+      initialNumToRender: 1,
+      windowSize: 1,
+      renderItem: ({item}) => <item value={item.key} sticky={item.sticky} />,
+      getItem: (data, index) => data[index],
+      getItemCount: data => data.length,
+      getItemLayout: (_, index) => ({
+        length: ITEM_HEIGHT,
+        offset: ITEM_HEIGHT * index,
+        index,
+      }),
+    };
+
+    let component;
+
+    ReactTestRenderer.act(() => {
+      component = ReactTestRenderer.create(
+        <VirtualizedList key={'A'} {...virtualizedListProps} />,
+      );
+    });
+
+    ReactTestRenderer.act(() => {
+      component
+        .getInstance()
+        ._onLayout({nativeEvent: {layout: {width: 10, height: 50}}});
+      component.getInstance()._onContentSizeChange(10, 100);
+      jest.runAllTimers();
+    });
+
+    expect(component).toMatchSnapshot();
+  });
+
+  it('keeps sticky headers realized after scrolled out of viewport', () => {
+    const items = Array(20)
+      .fill()
+      .map((_, i) =>
+        i % 3 === 0 ? {key: i, sticky: true} : {key: i, sticky: false},
+      );
+    const stickyIndices = items
+      .filter(item => item.sticky)
+      .map(item => item.key);
+
+    const ITEM_HEIGHT = 10;
+
+    const virtualizedListProps = {
+      data: items,
+      stickyHeaderIndices: stickyIndices,
+      initialNumToRender: 1,
+      windowSize: 1,
+      renderItem: ({item}) => <item value={item.key} sticky={item.sticky} />,
+      getItem: (data, index) => data[index],
+      getItemCount: data => data.length,
+      getItemLayout: (_, index) => ({
+        length: ITEM_HEIGHT,
+        offset: ITEM_HEIGHT * index,
+        index,
+      }),
+    };
+
+    let component;
+
+    ReactTestRenderer.act(() => {
+      component = ReactTestRenderer.create(
+        <VirtualizedList key={'A'} {...virtualizedListProps} />,
+      );
+    });
+
+    ReactTestRenderer.act(() => {
+      component
+        .getInstance()
+        ._onLayout({nativeEvent: {layout: {width: 10, height: 50}}});
+      component.getInstance()._onContentSizeChange(10, 200);
+      jest.runAllTimers();
+    });
+
+    ReactTestRenderer.act(() => {
+      component.getInstance()._onScroll({
+        nativeEvent: {
+          contentOffset: {x: 0, y: 150},
+          contentSize: {width: 10, height: 200},
+          layoutMeasurement: {width: 10, height: 50},
+        },
+      });
+      jest.runAllTimers();
+    });
+
+    expect(component).toMatchSnapshot();
+  });
 });