From e5acebabcbf8163642a633ee0d9fc696eec17fc1 Mon Sep 17 00:00:00 2001
From: Steven Kirk <grokys@users.noreply.github.com>
Date: Thu, 20 Apr 2023 18:20:49 +0200
Subject: [PATCH 1/2] Make thumb drag delta relative to root.

#10892 changed the thumb drag delta to be relative to the parent, but the problem was that is that if the thumb controls the position of the parent then the delta will be incorrect.

This was causing a bug in `TreeDataGrid` headers which were jumping around: the `Thumb` is a child of the header and causes the header to move.
---
 src/Avalonia.Controls/Primitives/Thumb.cs | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/src/Avalonia.Controls/Primitives/Thumb.cs b/src/Avalonia.Controls/Primitives/Thumb.cs
index 993d054f87..5dd2bd067b 100644
--- a/src/Avalonia.Controls/Primitives/Thumb.cs
+++ b/src/Avalonia.Controls/Primitives/Thumb.cs
@@ -85,7 +85,7 @@ namespace Avalonia.Controls.Primitives
         {
             if (_lastPoint.HasValue)
             {
-                var point = e.GetPosition(this.GetVisualParent());
+                var point = e.GetPosition((Visual?)this.GetVisualRoot());
                 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(this.GetVisualParent());
+            _lastPoint = e.GetPosition((Visual?)this.GetVisualRoot());
 
             var ev = new VectorEventArgs
             {
@@ -123,7 +123,7 @@ namespace Avalonia.Controls.Primitives
                 var ev = new VectorEventArgs
                 {
                     RoutedEvent = DragCompletedEvent,
-                    Vector = (Vector)e.GetPosition(this.GetVisualParent()),
+                    Vector = (Vector)e.GetPosition((Visual?)this.GetVisualRoot()),
                 };
 
                 RaiseEvent(ev);

From 5262eec4cf1f5134ac873a67a386bde9678913ab Mon Sep 17 00:00:00 2001
From: Steven Kirk <grokys@users.noreply.github.com>
Date: Fri, 21 Apr 2023 10:38:00 +0200
Subject: [PATCH 2/2] 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 | 4 ++--
 src/Avalonia.Controls/Primitives/Thumb.cs   | 6 +++---
 2 files changed, 5 insertions(+), 5 deletions(-)

diff --git a/src/Avalonia.Base/Input/PointerEventArgs.cs b/src/Avalonia.Base/Input/PointerEventArgs.cs
index 28a3c3aefb..beb953ce8f 100644
--- a/src/Avalonia.Base/Input/PointerEventArgs.cs
+++ b/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);
diff --git a/src/Avalonia.Controls/Primitives/Thumb.cs b/src/Avalonia.Controls/Primitives/Thumb.cs
index 5dd2bd067b..9854bdbea6 100644
--- a/src/Avalonia.Controls/Primitives/Thumb.cs
+++ b/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);