Fix InlineDictionary enumeration when an array is used

src/Avalonia.Base/Utilities/SmallDictionary.cs

@@ -51,7 +51,7 @@ internal struct InlineDictionary<TKey, TValue> : IEnumerable<KeyValuePair<TKey,
                         throw new ArgumentException("Key already exists in dictionary");
                 }
 
-                if (arr[c].Key == null)
+                if (arr[c].Key == null && free == -1)
                     free = c;
             }
 
@@ -337,11 +337,15 @@ internal struct InlineDictionary<TKey, TValue> : IEnumerable<KeyValuePair<TKey,
             }
             else if (_type == Type.Array)
             {
-                var next = _index + 1;
-                if (_arr!.Length - 1 < next || _arr[next].Key == null)
-                    return false;
-                _index = next;
-                return true;
+                for (var next = _index + 1; next < _arr!.Length; ++next)
+                {
+                    if (_arr[next].Key != null)
+                    {
+                        _index = next;
+                        return true;
+                    }
+                }
+                return false;
             }
             else if (_type == Type.Dictionary)
                 return _inner.MoveNext();

tests/Avalonia.Base.UnitTests/Utilities/InlineDictionaryTests.cs

@@ -0,0 +1,53 @@
+#nullable enable
+
+using System.Collections.Generic;
+using Avalonia.Utilities;
+using Xunit;
+
+namespace Avalonia.Base.UnitTests.Utilities;
+
+public class InlineDictionaryTests
+{
+    [Fact]
+    public void Enumeration_After_Add_With_Internal_Array_Works()
+    {
+        var dic = new InlineDictionary<string, int>();
+        dic.Add("foo", 1);
+        dic.Add("bar", 2);
+        dic.Add("baz", 3);
+
+        Assert.Equal(
+            new[] {
+                new KeyValuePair<string, int>("foo", 1),
+                new KeyValuePair<string, int>("bar", 2),
+                new KeyValuePair<string, int>("baz", 3)
+            },
+            dic);
+    }
+
+    [Fact]
+    public void Enumeration_After_Remove_With_Internal_Array_Works()
+    {
+        var dic = new InlineDictionary<string, int>();
+        dic.Add("foo", 1);
+        dic.Add("bar", 2);
+        dic.Add("baz", 3);
+
+        Assert.Equal(
+            new[] {
+                new KeyValuePair<string, int>("foo", 1),
+                new KeyValuePair<string, int>("bar", 2),
+                new KeyValuePair<string, int>("baz", 3)
+            },
+            dic);
+
+        dic.Remove("bar");
+
+        Assert.Equal(
+            new[] {
+                new KeyValuePair<string, int>("foo", 1),
+                new KeyValuePair<string, int>("baz", 3)
+            },
+            dic);
+    }
+}