fix: Fix StackVector not copying elements over when resizing

src/Uno.UI.Tests/Foundation/Given_StackVector.cs

@@ -73,5 +73,25 @@ public unsafe void ItemsInStackVector()
 				}
 			}
 		}
+
+		[TestMethod]
+		public void TestResizingShouldCopyOldElements()
+		{
+			var sut = new StackVector<int>(2);
+
+			for (int i = 0; i < 200; i++)
+			{
+				ref var item1 = ref sut.PushBack();
+				item1 = i;
+			}
+
+			sut.Count.Should().Be(200);
+			sut.Should().HaveCount(200);
+
+			for (int i = 0; i < 200; i++)
+			{
+				Assert.AreEqual(i, sut[i]);
+			}
+		}
 	}
 }

src/Uno.UWP/Collections/StackVector.cs

@@ -46,12 +46,17 @@ public void Resize(int newCount)
 
 		private void AllocateInner(int newSize)
 		{
+			var newArray = ArrayPool<T>.Shared.Rent(newSize);
+			var newMemory = new Memory<T>(newArray);
+			_inner.CopyTo(newMemory);
+
 			if (_originalArray != null)
 			{
 				ArrayPool<T>.Shared.Return(_originalArray, clearArray: true);
 			}
 
-			_inner = new Memory<T>(_originalArray = ArrayPool<T>.Shared.Rent(newSize));
+			_originalArray = newArray;
+			_inner = new Memory<T>(_originalArray);
 		}
 
 		public void Dispose()