From 08a8badd7afbf87c45d985de815b82b9d64fc6f0 Mon Sep 17 00:00:00 2001
From: Steven Kirk <grokys@gmail.com>
Date: Fri, 18 Sep 2020 00:28:56 +0200
Subject: [PATCH] Don't try to realize index -1.

`Algorithm_GetAnchorForTargetElement` can return -1 and when it does so we try to realize that item, which obviously doesn't exist, so boom.
---
 src/Avalonia.Layout/FlowLayoutAlgorithm.cs | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/Avalonia.Layout/FlowLayoutAlgorithm.cs b/src/Avalonia.Layout/FlowLayoutAlgorithm.cs
index cd7f725f18..eace54d2e0 100644
--- a/src/Avalonia.Layout/FlowLayoutAlgorithm.cs
+++ b/src/Avalonia.Layout/FlowLayoutAlgorithm.cs
@@ -211,7 +211,7 @@ namespace Avalonia.Layout
                             anchorPosition = new Point(anchorBounds.X, anchorBounds.Y);
                         }
                     }
-                    else
+                    else if (anchorIndex >= 0)
                     {
                         // It is possible to end up in a situation during a collection change where GetAnchorForTargetElement returns an index
                         // which is not in the realized range. Eg. insert one item at index 0 for a grid layout.