Fix InlineDictionary Behavior for Single Item Reassignment (#17370)

* Fix InlineDictionary Behavior for Single Item Reassignment

Previous Behavior:

- When the `Set` method was called, it ignored the `overwrite` parameter if the dictionary contained only one item.
- This caused the `Set` method to behave like the `Add` method, leading to unexpected behavior in composition animations.
- Specifically, the second composition animation would not play because it could not replace the first animation. Instead, it was added after the first animation, preventing it from being executed.

Updated Behavior:

- The `InlineDictionary` now respects the `overwrite` parameter even when it contains only one item.
- This ensures that the second composition animation can overwrite the first one, allowing it to play correctly.

* Append unit test for InlineDictionary

* keep the `(TKey)_data` in a variable and reuse it below line 90 so we avoid a double cast

* Rename the test method

* Remove the unnecessary assignment code.

src/Avalonia.Base/Utilities/SmallDictionary.cs

@@ -77,9 +77,17 @@ void SetCore(TKey key, TValue value, bool overwrite)
         }
         else
         {
-            // We have a single element, upgrade to array
+            // We have a single element, check if we should update the value.
+            var data = (TKey)_data;
+            if (data == key && overwrite)
+            {
+                _value = value;
+
+                return;
+            }
+            // If we do not replace it, upgrade to array.
             arr = new KeyValuePair[6];
-            arr[0] = new KeyValuePair((TKey)_data, _value);
+            arr[0] = new KeyValuePair(data, _value);
             arr[1] = new KeyValuePair(key, value);
             _data = arr;
             _value = default;

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

@@ -8,6 +8,17 @@ namespace Avalonia.Base.UnitTests.Utilities;
 
 public class InlineDictionaryTests
 {
+    [Fact]
+    public void Set_Twice_With_Single_Item_Works()
+    {
+        var dic = new InlineDictionary<string, int>();
+        dic["foo"] = 1;
+        Assert.Equal(1, dic["foo"]);
+
+        dic["foo"] = 2;
+        Assert.Equal(2, dic["foo"]);
+    }
+
     [Fact]
     public void Enumeration_After_Add_With_Internal_Array_Works()
     {