Fix TranslatePoint.

- Make `VisualExtensions.TranslatePoint` respect render transforms.
- Move `IVisual.TransformToVisual` out of the interface and into `VisualExtensions` as an extension method
- Make `TranslatePoint` return a nullable value as `TransformToVisual` does, so it can return null when controls don't have a common ancestor

src/Avalonia.Input/DragDropDevice.cs

@@ -23,7 +23,13 @@ namespace Avalonia.Input
         {
             if (target == null)
                 return DragDropEffects.None;
-            var args = new DragEventArgs(routedEvent, data, target, inputRoot.TranslatePoint(point, target), modifiers)
+
+            var p = inputRoot.TranslatePoint(point, target);
+
+            if (!p.HasValue)
+                return DragDropEffects.None;
+
+            var args = new DragEventArgs(routedEvent, data, target, p.Value, modifiers)
             {
                 RoutedEvent = routedEvent,
                 DragEffects = operation
@@ -108,4 +114,4 @@ namespace Avalonia.Input
             }
         }
     }
-}
+}

src/Avalonia.Input/DragEventArgs.cs

@@ -26,8 +26,9 @@ namespace Avalonia.Input
 
             if (_target != null)
             {
-                point = _target.TranslatePoint(_targetLocation, relativeTo);
+                point = _target.TranslatePoint(_targetLocation, relativeTo) ?? point;
             }
+
             return point;
         }
 
@@ -41,4 +42,4 @@ namespace Avalonia.Input
         }
 
     }
-}
+}

src/Avalonia.Visuals/Rendering/SceneGraph/Scene.cs

@@ -173,7 +173,7 @@ namespace Avalonia.Rendering.SceneGraph
                 if (node.GeometryClip != null)
                 {
                     var controlPoint = Root.Visual.TranslatePoint(p, node.Visual);
-                    clipped = !node.GeometryClip.FillContains(controlPoint);
+                    clipped = !node.GeometryClip.FillContains(controlPoint.Value);
                 }
 
                 if (!clipped)

src/Avalonia.Visuals/Visual.cs

@@ -291,29 +291,6 @@ namespace Avalonia
             Contract.Requires<ArgumentNullException>(context != null);
         }
 
-        /// <summary>
-        /// Returns a transform that transforms the visual's coordinates into the coordinates
-        /// of the specified <paramref name="visual"/>.
-        /// </summary>
-        /// <param name="visual">The visual to translate the coordinates to.</param>
-        /// <returns>
-        /// A <see cref="Matrix"/> containing the transform or null if the visuals don't share a
-        /// common ancestor.
-        /// </returns>
-        public Matrix? TransformToVisual(IVisual visual)
-        {
-            var common = this.FindCommonVisualAncestor(visual);
-
-            if (common != null)
-            {
-                var thisOffset = GetOffsetFrom(common, this);
-                var thatOffset = GetOffsetFrom(common, visual);
-                return -thatOffset * thisOffset;
-            }
-
-            return null;
-        }
-
         /// <summary>
         /// Indicates that a property change should cause <see cref="InvalidateVisual"/> to be
         /// called.
@@ -480,45 +457,6 @@ namespace Avalonia
             }
         }
 
-        /// <summary>
-        /// Gets the visual offset from the specified ancestor.
-        /// </summary>
-        /// <param name="ancestor">The ancestor visual.</param>
-        /// <param name="visual">The visual.</param>
-        /// <returns>The visual offset.</returns>
-        private static Matrix GetOffsetFrom(IVisual ancestor, IVisual visual)
-        {
-            var result = Matrix.Identity;
-
-            while (visual != ancestor)
-            {
-                if (visual.RenderTransform?.Value != null)
-                {
-                    var origin = visual.RenderTransformOrigin.ToPixels(visual.Bounds.Size);
-                    var offset = Matrix.CreateTranslation(origin);
-                    var renderTransform = (-offset) * visual.RenderTransform.Value * (offset);
-
-                    result *= renderTransform;
-                }
-
-                var topLeft = visual.Bounds.TopLeft;
-
-                if (topLeft != default)
-                {
-                    result *= Matrix.CreateTranslation(topLeft);
-                }
-
-                visual = visual.VisualParent;
-
-                if (visual == null)
-                {
-                    throw new ArgumentException("'visual' is not a descendant of 'ancestor'.");
-                }
-            }
-
-            return result;
-        }
-
         /// <summary>
         /// Called when a visual's <see cref="RenderTransform"/> changes.
         /// </summary>

src/Avalonia.Visuals/VisualExtensions.cs

@@ -2,7 +2,6 @@
 // Licensed under the MIT license. See licence.md file in the project root for full license information.
 
 using System;
-using Avalonia.Rendering;
 using Avalonia.VisualTree;
 
 namespace Avalonia
@@ -20,10 +19,8 @@ namespace Avalonia
         /// <returns>The point in client coordinates.</returns>
         public static Point PointToClient(this IVisual visual, PixelPoint point)
         {
-            var (root, offset) = GetRootAndPosition(visual);
-            var screenOffset = PixelPoint.FromPoint((Point)offset, root.RenderScaling);
-            var screenPoint = new PixelPoint(point.X - screenOffset.X, point.Y - screenOffset.Y);
-            return root.PointToClient(screenPoint);
+            var rootPoint = visual.VisualRoot.PointToClient(point);
+            return visual.VisualRoot.TranslatePoint(rootPoint, visual).Value;
         }
 
         /// <summary>
@@ -34,48 +31,93 @@ namespace Avalonia
         /// <returns>The point in screen coordinates.</returns>
         public static PixelPoint PointToScreen(this IVisual visual, Point point)
         {
-            var p = GetRootAndPosition(visual);
-            return p.Item1.PointToScreen(point + p.Item2);
+            var p = visual.TranslatePoint(point, visual.VisualRoot);
+            return visual.VisualRoot.PointToScreen(p.Value);
+        }
+
+        /// <summary>
+        /// Returns a transform that transforms the visual's coordinates into the coordinates
+        /// of the specified <paramref name="to"/>.
+        /// </summary>
+        /// <param name="from">The visual whose coordinates are to be transformed.</param>
+        /// <param name="to">The visual to translate the coordinates to.</param>
+        /// <returns>
+        /// A <see cref="Matrix"/> containing the transform or null if the visuals don't share a
+        /// common ancestor.
+        /// </returns>
+        public static Matrix? TransformToVisual(this IVisual from, IVisual to)
+        {
+            var common = from.FindCommonVisualAncestor(to);
+
+            if (common != null)
+            {
+                var thisOffset = GetOffsetFrom(common, from);
+                var thatOffset = GetOffsetFrom(common, to);
+                return -thatOffset * thisOffset;
+            }
+
+            return null;
         }
 
         /// <summary>
         /// Translates a point relative to this visual to coordinates that are relative to the specified visual.
-        /// The visual and relativeTo should be descendants of the same root window
         /// </summary>
         /// <param name="visual">The visual.</param>
         /// <param name="point">The point value, as relative to this visual.</param>
         /// <param name="relativeTo">The visual to translate the given point into.</param>
-        /// <returns>A point value, now relative to the target visual rather than this source element.</returns>
-        public static Point TranslatePoint(this IVisual visual, Point point, IVisual relativeTo)
+        /// <returns>
+        /// A point value, now relative to the target visual rather than this source element, or null if the
+        /// two elements have no common ancestor.
+        /// </returns>
+        public static Point? TranslatePoint(this IVisual visual, Point point, IVisual relativeTo)
         {
-            var pos = GetRootAndPosition(visual);
-            var relToPos = GetRootAndPosition(relativeTo);
+            var transform = visual.TransformToVisual(relativeTo);
 
-            return point - (relToPos.Item2 - pos.Item2);
+            if (transform.HasValue)
+            {
+                return point.Transform(transform.Value);
+            }
+
+            return null;
         }
 
         /// <summary>
-        /// Gets the root of the control's visual tree and the position of the control 
-        /// in the root's coordinate space.
+        /// Gets a transform from an ancestor to a descendent.
         /// </summary>
-        /// <param name="v">The visual.</param>
-        /// <returns>A tuple containing the root and the position of the control.</returns>
-        private static Tuple<IRenderRoot, Vector> GetRootAndPosition(IVisual v)
+        /// <param name="ancestor">The ancestor visual.</param>
+        /// <param name="visual">The visual.</param>
+        /// <returns>The transform.</returns>
+        private static Matrix GetOffsetFrom(IVisual ancestor, IVisual visual)
         {
-            var result = new Vector();
+            var result = Matrix.Identity;
 
-            while (!(v is IRenderRoot))
+            while (visual != ancestor)
             {
-                result = new Vector(result.X + v.Bounds.X, result.Y + v.Bounds.Y);
-                v = v.VisualParent;
+                if (visual.RenderTransform?.Value != null)
+                {
+                    var origin = visual.RenderTransformOrigin.ToPixels(visual.Bounds.Size);
+                    var offset = Matrix.CreateTranslation(origin);
+                    var renderTransform = (-offset) * visual.RenderTransform.Value * (offset);
+
+                    result *= renderTransform;
+                }
+
+                var topLeft = visual.Bounds.TopLeft;
+
+                if (topLeft != default)
+                {
+                    result *= Matrix.CreateTranslation(topLeft);
+                }
+
+                visual = visual.VisualParent;
 
-                if (v == null)
+                if (visual == null)
                 {
-                    throw new InvalidOperationException("Control is not attached to visual tree.");
+                    throw new ArgumentException("'visual' is not a descendant of 'ancestor'.");
                 }
             }
 
-            return Tuple.Create((IRenderRoot)v, result);
+            return result;
         }
     }
 }

src/Avalonia.Visuals/VisualTree/IVisual.cs

@@ -116,16 +116,5 @@ namespace Avalonia.VisualTree
         /// </summary>
         /// <param name="context">The context.</param>
         void Render(DrawingContext context);
-
-        /// <summary>
-        /// Returns a transform that transforms the visual's coordinates into the coordinates
-        /// of the specified <paramref name="visual"/>.
-        /// </summary>
-        /// <param name="visual">The visual to translate the coordinates to.</param>
-        /// <returns>
-        /// A <see cref="Matrix"/> containing the transform or null if the visuals don't share a
-        /// common ancestor.
-        /// </returns>
-        Matrix? TransformToVisual(IVisual visual);
     }
 }

src/Avalonia.Visuals/VisualTree/VisualExtensions.cs

@@ -132,8 +132,14 @@ namespace Avalonia.VisualTree
             Contract.Requires<ArgumentNullException>(visual != null);
 
             var root = visual.GetVisualRoot();
-            p = visual.TranslatePoint(p, root);
-            return root.Renderer.HitTest(p, visual, filter);
+            var rootPoint = visual.TranslatePoint(p, root);
+
+            if (rootPoint.HasValue)
+            {
+                return root.Renderer.HitTest(rootPoint.Value, visual, filter);
+            }
+
+            return Enumerable.Empty<IVisual>();
         }
 
         /// <summary>