Fix problems if _updateCellsToRender gets called too early

Summary:
(prior to the scroll metrics being set up, or prior to the view being scrolled to the right location).

I'm using a StackNavigator to push a FlatList secondary screen, and it was causing extraneous rendering (item X ->item 0 -> item X)

I've logged inside the render functions of my `renderItem` function, and verified that it no longer attempts to render item 0 anymore. And I've verified from the `VirtualizedList.state` no longer renders a `{first: 0, last: 0}`, but persistently stays at `{first: X, last: X}` from start until things stabilize.
Closes #14562

Reviewed By: bvaughn

Differential Revision: D5283771

Pulled By: sahrens

fbshipit-source-id: 0f70ac0b89922449bd20bfa69edbc8939eafdf1f

Libraries/Lists/VirtualizedList.js

@@ -1026,12 +1026,24 @@ class VirtualizedList extends React.PureComponent<OptionalProps, Props, State> {
     this.setState(state => {
       let newState;
       if (!isVirtualizationDisabled) {
-        newState = computeWindowedRenderLimits(
-          this.props,
-          state,
-          this._getFrameMetricsApprox,
-          this._scrollMetrics,
-        );
+        // If we run this with bogus data, we'll force-render window {first: 0, last: 0},
+        // and wipe out the initialNumToRender rendered elements.
+        // So let's wait until the scroll view metrics have been set up. And until then,
+        // we will trust the initialNumToRender suggestion
+        if (this._scrollMetrics.visibleLength) {
+          // If we have a 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.
+          if (!this.props.initialScrollIndex || this._scrollMetrics.offset) {
+            newState = computeWindowedRenderLimits(
+              this.props,
+              state,
+              this._getFrameMetricsApprox,
+              this._scrollMetrics,
+            );
+          }
+        }
       } else {
         const {contentLength, offset, visibleLength} = this._scrollMetrics;
         const distanceFromEnd = contentLength - visibleLength - offset;