From a9c2ae7cb35424e109c4c017d634c9e1a314ea04 Mon Sep 17 00:00:00 2001
From: ptaylor <paul.e.taylor@me.com>
Date: Sun, 27 Jan 2019 12:35:58 -0800
Subject: [PATCH] Guard against incomplete cell measurement

---
 source/Grid/utils/CellSizeAndPositionManager.js | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/source/Grid/utils/CellSizeAndPositionManager.js b/source/Grid/utils/CellSizeAndPositionManager.js
index 1f37930d1..74a812b60 100644
--- a/source/Grid/utils/CellSizeAndPositionManager.js
+++ b/source/Grid/utils/CellSizeAndPositionManager.js
@@ -246,6 +246,13 @@ export default class CellSizeAndPositionManager {
         return nearestCellIndex;
       }
       nearestCellIndex = vector.indexOf(targetOffset);
+      // Guard in case `getSizeAndPositionOfCell` didn't fully measure to
+      // the nearestCellIndex. This might happen scrolling quickly down
+      // and back up on large lists -- possible race with React or DOM?
+      if (nearestCellIndex === -1) {
+        nearestCellIndex = this._lastMeasuredIndex;
+        this._lastMeasuredIndex = nearestCellIndex - 1;
+      }
     }
 
     return nearestCellIndex;