Added ThumbAutomationPeer.

To make slider thumb visible to automation.

src/Avalonia.Controls/Automation/Peers/ThumbAutomationPeer.cs

@@ -0,0 +1,12 @@
+using Avalonia.Automation.Peers;
+using Avalonia.Controls.Primitives;
+
+namespace Avalonia.Controls.Automation.Peers
+{
+    public class ThumbAutomationPeer : ControlAutomationPeer
+    {
+        public ThumbAutomationPeer(Thumb owner) : base(owner) { }
+        protected override AutomationControlType GetAutomationControlTypeCore() => AutomationControlType.Thumb;
+        protected override bool IsContentElementCore() => false;
+    }
+}

src/Avalonia.Controls/Primitives/Thumb.cs

@@ -1,4 +1,6 @@
 using System;
+using Avalonia.Automation.Peers;
+using Avalonia.Controls.Automation.Peers;
 using Avalonia.Controls.Metadata;
 using Avalonia.Input;
 using Avalonia.Interactivity;
@@ -45,6 +47,8 @@ namespace Avalonia.Controls.Primitives
             remove { RemoveHandler(DragCompletedEvent, value); }
         }
 
+        protected override AutomationPeer OnCreateAutomationPeer() => new ThumbAutomationPeer(this);
+
         protected virtual void OnDragStarted(VectorEventArgs e)
         {
         }