fix

src/Avalonia.Base/Collections/AvaloniaDictionary.cs

@@ -146,6 +146,7 @@ namespace Avalonia.Collections
         {
             if (_inner.TryGetValue(key, out var value))
             {
+                _inner.Remove(key);
                 PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(nameof(Count)));
                 PropertyChanged?.Invoke(this, new PropertyChangedEventArgs($"Item[{key}]"));
 

tests/Avalonia.Base.UnitTests/Collections/AvaloniaDictionaryTests.cs

@@ -1,3 +1,4 @@
+using System;
 using System.Collections.Generic;
 using System.Collections.Specialized;
 using Avalonia.Collections;
@@ -104,6 +105,16 @@ namespace Avalonia.Base.UnitTests.Collections
             Assert.Equal(new KeyValuePair<string, string>("foo", "bar"), tracker.Args.OldItems[0]);
         }
 
+        [Fact]
+        public void Remove_Method_Should_Remove_Item_From_Collection()
+        {
+            var target = new AvaloniaDictionary<string, string>() { { "foo", "bar" } };
+            Assert.Equal(target.Count, 1);
+
+            target.Remove("foo");
+            Assert.Equal(target.Count, 0);
+        }
+
         [Fact]
         public void Removing_Item_Should_Raise_PropertyChanged()
         {