fix: Fix Slider thumb not visible when Value is at maximum

There was a bug occurring on iOS where the indicator Rectangle was having its size set from within SizeChanged before the Thumb had a chance to be arranged, leaving it to take the entire space and leave none for the Thumb.

As a workaround, return the Width/Height if ActualWidth/ActualHeight haven't been set yet.

src/Uno.UI.RuntimeTests/Tests/Windows_UI_Xaml_Controls/Given_Slider.cs

@@ -0,0 +1,42 @@
+using System;
+using System.Collections.Generic;
+using System.Text;
+using System.Threading.Tasks;
+using Microsoft.VisualStudio.TestTools.UnitTesting;
+using Windows.UI.Xaml;
+using Windows.UI.Xaml.Controls;
+using Windows.UI.Xaml.Controls.Primitives;
+using static Private.Infrastructure.TestServices;
+#if NETFX_CORE
+using Uno.UI.Extensions;
+#elif __IOS__
+using UIKit;
+#elif __MACOS__
+using AppKit;
+#else
+using Uno.UI;
+#endif
+
+namespace Uno.UI.RuntimeTests.Tests.Windows_UI_Xaml_Controls
+{
+	[TestClass]
+	[RunsOnUIThread]
+	public class Given_Slider
+	{
+		[TestMethod]
+		public async Task When_Value_At_Maximum()
+		{
+			var slider = new Slider { Minimum = 0, Maximum = 100, Value = 100, Orientation = Orientation.Horizontal, Width = 320, VerticalAlignment = VerticalAlignment.Stretch };
+			WindowHelper.WindowContent = slider;
+
+			await WindowHelper.WaitForIdle();
+
+			var thumb = slider.FindFirstChild<Thumb>();
+
+			Assert.IsNotNull(thumb);
+
+			Assert.IsTrue(thumb.ActualWidth > 0);
+			Assert.IsTrue(thumb.ActualHeight > 0);
+		}
+	}
+}

src/Uno.UI/UI/Xaml/Controls/Slider/Slider.cs

@@ -278,20 +278,44 @@ private void ApplyValueToSlide()
 			{
 				if (_horizontalThumb != null && _horizontalDecreaseRect != null)
 				{
-					var maxWidth = ActualWidth - _horizontalThumb.ActualWidth;
+					var maxWidth = ActualWidth - GetHorizontalThumbWidth();
 					_horizontalDecreaseRect.Width = (float)((Value - Minimum) / (Maximum - Minimum)) * maxWidth;
 				}
 			}
 			else
 			{
 				if (_verticalThumb != null && _verticalDecreaseRect != null)
 				{
-					var maxHeight = ActualHeight - _verticalThumb.ActualHeight;
+					var maxHeight = ActualHeight - GetVerticalThumbHeight();
 					_verticalDecreaseRect.Height = (float)((Value - Minimum) / (Maximum - Minimum)) * maxHeight;
 				}
 			}
 		}
 
+		private double GetHorizontalThumbWidth()
+		{
+			// In some cases, because of the timing of SizeChanged, this may be called before the thumb has been measured. If so, we rely on the fact
+			// that the dimensions are hard-coded by the UWP and Fluent styles.
+			if (_horizontalThumb.ActualWidth == 0 && !double.IsNaN(_horizontalThumb.Width))
+			{
+				return _horizontalThumb.Width;
+			}
+
+			return _horizontalThumb.ActualWidth;
+		}
+
+		private double GetVerticalThumbHeight()
+		{
+			// In some cases, because of the timing of SizeChanged, this may be called before the thumb has been measured. If so, we rely on the fact
+			// that the dimensions are hard-coded by the UWP and Fluent styles.
+			if (_verticalThumb.ActualHeight == 0 && !double.IsNaN(_verticalThumb.Height))
+			{
+				return _verticalThumb.Height;
+			}
+
+			return _verticalThumb.ActualHeight;
+		}
+
 		/// <summary>
 		/// Get the snap frequency, given the current values of SnapsTo,
 		/// StepFrequency and TickFrequency, and sanitizing the value