Passing null gives us the point relative to the root.

And update the documentation for `GetPosition` to explain what `null` does (as in `GetCurrentPoint`).

src/Avalonia.Base/Input/PointerEventArgs.cs

@@ -77,14 +77,14 @@ namespace Avalonia.Input
         /// <summary>
         /// Gets the pointer position relative to a control.
         /// </summary>
-        /// <param name="relativeTo">The control.</param>
+        /// <param name="relativeTo">The visual whose coordinate system to use. Pass null for toplevel coordinate system</param>
         /// <returns>The pointer position in the control's coordinates.</returns>
         public Point GetPosition(Visual? relativeTo) => GetPosition(_rootVisualPosition, relativeTo);
 
         /// <summary>
         /// Returns the PointerPoint associated with the current event
         /// </summary>
-        /// <param name="relativeTo">The visual which coordinate system to use. Pass null for toplevel coordinate system</param>
+        /// <param name="relativeTo">The visual whose coordinate system to use. Pass null for toplevel coordinate system</param>
         /// <returns></returns>
         public PointerPoint GetCurrentPoint(Visual? relativeTo)
             => new PointerPoint(Pointer, GetPosition(relativeTo), _properties);

src/Avalonia.Controls/Primitives/Thumb.cs

@@ -85,7 +85,7 @@ namespace Avalonia.Controls.Primitives
         {
             if (_lastPoint.HasValue)
             {
-                var point = e.GetPosition((Visual?)this.GetVisualRoot());
+                var point = e.GetPosition(null);
                 var ev = new VectorEventArgs
                 {
                     RoutedEvent = DragDeltaEvent,
@@ -100,7 +100,7 @@ namespace Avalonia.Controls.Primitives
         protected override void OnPointerPressed(PointerPressedEventArgs e)
         {
             e.Handled = true;
-            _lastPoint = e.GetPosition((Visual?)this.GetVisualRoot());
+            _lastPoint = e.GetPosition(null);
 
             var ev = new VectorEventArgs
             {
@@ -123,7 +123,7 @@ namespace Avalonia.Controls.Primitives
                 var ev = new VectorEventArgs
                 {
                     RoutedEvent = DragCompletedEvent,
-                    Vector = (Vector)e.GetPosition((Visual?)this.GetVisualRoot()),
+                    Vector = (Vector)e.GetPosition(null),
                 };
 
                 RaiseEvent(ev);