fix(tabbar): initial orientation and selection not working

src/Uno.Toolkit.UI/Controls/TabBar/TabBar.cs

@@ -2,6 +2,7 @@
 using System.Collections;
 using System.Collections.Generic;
 using System.Collections.ObjectModel;
+using System.Diagnostics;
 using System.Linq;
 using System.Numerics;
 using Uno.Disposables;
@@ -195,6 +196,14 @@ private void OnLoaded(object sender, RoutedEventArgs e)
 			UpdateOrientation();
 		}
 
+		internal void OnItemsPanelConnected(TabBarListPanel panel)
+		{
+			Debug.Assert(ItemsPanelRoot != null, "ItemsPanelRoot is expected to be already set in here.");
+
+			SynchronizeInitialSelection();
+			UpdateOrientation();
+		}
+
 		private void OnTabBarItemClick(object sender, RoutedEventArgs e)
 		{
 			if (_isSynchronizingSelection)
@@ -441,7 +450,7 @@ private void RaiseSelectionChangedEvent(object? prevItem, object? nextItem)
 			return null;
 		}
 
-		private bool IsReady => _isLoaded && HasItems;
+		private bool IsReady => _isLoaded && HasItems && ItemsPanelRoot is { };
 
 		private bool HasItems => this.GetItems().Any();
 	}

src/Uno.Toolkit.UI/Controls/TabBar/TabBarListPanel.cs

@@ -17,6 +17,7 @@ namespace Uno.Toolkit.UI
 {
 	public partial class TabBarListPanel : Panel
 	{
+		#region Orientation
 		public Orientation Orientation
 		{
 			get { return (Orientation)GetValue(OrientationProperty); }
@@ -28,6 +29,20 @@ public Orientation Orientation
 			typeof(Orientation),
 			typeof(TabBarListPanel),
 			new PropertyMetadata(Orientation.Horizontal, (s, e) => ((TabBarListPanel)s).OnPropertyChanged(e)));
+		#endregion
+
+		public TabBarListPanel()
+		{
+			this.Loaded += OnLoaded;
+		}
+
+		private void OnLoaded(object sender, RoutedEventArgs e)
+		{
+			var owner = this.FindFirstParent<TabBar>();
+
+			// workaround for #1287 ItemsPanelRoot resolution timing related issue
+			owner?.OnItemsPanelConnected(this);
+		}
 
 		private void OnPropertyChanged(DependencyPropertyChangedEventArgs args)
 		{