Revert thumb to using relative coordinates for drag.

Undoes the changes in #10892 and #11087.

src/Avalonia.Controls/Primitives/Thumb.cs

@@ -4,7 +4,6 @@ using Avalonia.Controls.Automation.Peers;
 using Avalonia.Controls.Metadata;
 using Avalonia.Input;
 using Avalonia.Interactivity;
-using Avalonia.VisualTree;
 
 namespace Avalonia.Controls.Primitives
 {
@@ -85,22 +84,20 @@ namespace Avalonia.Controls.Primitives
         {
             if (_lastPoint.HasValue)
             {
-                var point = e.GetPosition(null);
                 var ev = new VectorEventArgs
                 {
                     RoutedEvent = DragDeltaEvent,
-                    Vector = point - _lastPoint.Value,
+                    Vector = e.GetPosition(this) - _lastPoint.Value,
                 };
 
                 RaiseEvent(ev);
-                _lastPoint = point;
             }
         }
 
         protected override void OnPointerPressed(PointerPressedEventArgs e)
         {
             e.Handled = true;
-            _lastPoint = e.GetPosition(null);
+            _lastPoint = e.GetPosition(this);
 
             var ev = new VectorEventArgs
             {
@@ -123,7 +120,7 @@ namespace Avalonia.Controls.Primitives
                 var ev = new VectorEventArgs
                 {
                     RoutedEvent = DragCompletedEvent,
-                    Vector = (Vector)e.GetPosition(null),
+                    Vector = (Vector)e.GetPosition(this),
                 };
 
                 RaiseEvent(ev);