Fix slider behaviour for bigger thumbs

Thumb now moves synchronously with pointer instead of relativ to track position.

src/Avalonia.Controls/Slider.cs

@@ -331,16 +331,17 @@ private void TrackPressed(object sender, PointerPressedEventArgs e)
             }
         }
 
-        private void MoveToPoint(PointerPoint x)
+        private void MoveToPoint(PointerPoint posOnTrack)
         {
             var orient = Orientation == Orientation.Horizontal;
-
-            var pointDen = orient ? _track.Bounds.Width : _track.Bounds.Height;
-            // Just add epsilon to avoid NaN in case 0/0
-            pointDen += double.Epsilon;
-
-            var pointNum = orient ? x.Position.X : x.Position.Y;
-            var logicalPos = MathUtilities.Clamp(pointNum / pointDen, 0.0d, 1.0d);
+            var thumbLength = (orient 
+                ? _track.Thumb.Bounds.Width 
+                : _track.Thumb.Bounds.Height) + double.Epsilon;
+            var trackLength = (orient 
+                ? _track.Bounds.Width 
+                : _track.Bounds.Height) - thumbLength;
+            var trackPos = orient ? posOnTrack.Position.X : posOnTrack.Position.Y;
+            var logicalPos = MathUtilities.Clamp((trackPos - thumbLength * 0.5) / trackLength, 0.0d, 1.0d);
             var invert = orient ? 
                 IsDirectionReversed ? 1 : 0 :
                 IsDirectionReversed ? 0 : 1;