fix: make sure initialScrollIndex is bigger than 0 (facebook#36844)

Summary:
Hey,

`adjustCellsAroundViewport` function was checking if `props.initialScrollIndex` is truthy and -1 was returning true. This caused bugs with rendering for tvOS: react-native-tvos/react-native-tvos#485 There are warnings in the code about `initalScrollIndex` being smaller than 0 but this if statement would still allow that.

## Changelog:

[General] [Fixed] - Make sure initialScrollToIndex is bigger than 0 when adjusting cells

Pull Request resolved: facebook#36844

Test Plan: Pass -1 as initialScrollToIndex. Check that this code is executed.

Reviewed By: cipolleschi

Differential Revision: D44856266

Pulled By: NickGerleman

fbshipit-source-id: 781a1c0efeae93f00766eede4a42559dcd066d7d

Author: okwasniewski

packages/virtualized-lists/Lists/VirtualizedList.js

@@ -616,7 +616,7 @@ class VirtualizedList extends StateSafePureComponent<Props, State> {
         ),
       };
     } else {
-      // If we have a non-zero initialScrollIndex and run this before we've scrolled,
+      // If we have a positive non-zero initialScrollIndex and run this before we've scrolled,
       // we'll wipe out the initialNumToRender rendered elements starting at initialScrollIndex.
       // So let's wait until we've scrolled the view to the right place. And until then,
       // we will trust the initialScrollIndex suggestion.
@@ -627,7 +627,8 @@ class VirtualizedList extends StateSafePureComponent<Props, State> {
       // - initialScrollIndex > 0 AND the end of the list is visible (this handles the case
       //   where the list is shorter than the visible area)
       if (
-        props.initialScrollIndex &&
+        props.initialScrollIndex != null &&
+        props.initialScrollIndex > 0 &&
         !this._scrollMetrics.offset &&
         Math.abs(distanceFromEnd) >= Number.EPSILON
       ) {

packages/virtualized-lists/Lists/__tests__/__snapshots__/VirtualizedList-test.js.snap

@@ -3122,12 +3122,53 @@ exports[`gracefully handles negative initialScrollIndex 1`] = `
       />
     </View>
     <View
-      style={
-        Object {
-          "height": 60,
-        }
-      }
-    />
+      onFocusCapture={[Function]}
+      style={null}
+    >
+      <MockCellItem
+        value={4}
+      />
+    </View>
+    <View
+      onFocusCapture={[Function]}
+      style={null}
+    >
+      <MockCellItem
+        value={5}
+      />
+    </View>
+    <View
+      onFocusCapture={[Function]}
+      style={null}
+    >
+      <MockCellItem
+        value={6}
+      />
+    </View>
+    <View
+      onFocusCapture={[Function]}
+      style={null}
+    >
+      <MockCellItem
+        value={7}
+      />
+    </View>
+    <View
+      onFocusCapture={[Function]}
+      style={null}
+    >
+      <MockCellItem
+        value={8}
+      />
+    </View>
+    <View
+      onFocusCapture={[Function]}
+      style={null}
+    >
+      <MockCellItem
+        value={9}
+      />
+    </View>
   </View>
 </RCTScrollView>
 `;