test: add When_StateTriggers_Evaluated_Before_First_Layout

src/Uno.UI.RuntimeTests/Tests/Windows_UI_Xaml/Given_VisualStateManager.cs

@@ -1,13 +1,16 @@
 using System;
+using System.Collections.Generic;
 using System.Linq;
 using System.Threading.Tasks;
+using Windows.Foundation;
 using Microsoft.UI.Xaml;
 using Microsoft.UI.Xaml.Controls;
 using Microsoft.UI.Xaml.Controls.Primitives;
 using Microsoft.UI.Xaml.Media;
 using Microsoft.UI.Xaml.Shapes;
 using Microsoft.VisualStudio.TestTools.UnitTesting;
 using MUXControlsTestApp.Utilities;
+using SamplesApp.UITests;
 using Uno.Extensions;
 using Uno.UI.Extensions;
 using Uno.UI.RuntimeTests.Helpers;
@@ -79,4 +82,65 @@ public async Task SelectorItem_SelectedState()
 		var states = VisualStateHelper.GetCurrentVisualStateName(container2).ToArray();
 		Assert.IsTrue(states.Contains("MultiSelectEnabled"), $"container2 is not in 'MultiSelectEnabled' state: states={states.JoinBy(",")}");
 	}
+
+#if HAS_UNO
+	[TestMethod]
+	[UnoWorkItem("https://github.com/unoplatform/uno/issues/19364")]
+	public async Task When_StateTriggers_Evaluated_Before_First_Layout()
+	{
+		MyUserControl uc = new MyUserControl();
+		VisualStateManager.SetVisualStateGroups(uc, new List<VisualStateGroup>
+		{
+			new VisualStateGroup()
+			{
+				States =
+				{
+					new VisualState
+					{
+						Name = "MyVisualState1",
+					},
+					new VisualState
+					{
+						Name = "MyVisualState2",
+						StateTriggers =
+						{
+							new AdaptiveTrigger()
+							{
+								MinWindowWidth = 1
+							}
+						}
+					}
+				}
+			}
+		});
+
+		var contentControl = new ContentControl
+		{
+			Content = "0",
+			ContentTemplate = new DataTemplate(() =>
+			{
+				return uc;
+			})
+		};
+
+		await UITestHelper.Load(contentControl, control => control.IsLoaded);
+		Assert.AreEqual("MyVisualState2", uc.VisualStateOnFirstMeasure?.Name);
+	}
+
+	private partial class MyUserControl : UserControl
+	{
+		private bool _firstMeasure = true;
+		public VisualState VisualStateOnFirstMeasure { get; set; }
+
+		protected override Size MeasureOverride(Size availableSize)
+		{
+			if (_firstMeasure)
+			{
+				_firstMeasure = false;
+				VisualStateOnFirstMeasure = VisualStateManager.GetVisualStateGroups(this)[0].CurrentState;
+			}
+			return base.MeasureOverride(availableSize);
+		}
+	}
+#endif
 }