[Scrollbar] dont try and arrange the thumb and buttons if the extent is zero.

src/Avalonia.Controls/Primitives/Track.cs

@@ -137,14 +137,17 @@ namespace Avalonia.Controls.Primitives
                     decreaseButton.Arrange(new Rect(0, 0, firstWidth, finalSize.Height));
                 }
 
-                if (thumb != null)
+                if (!double.IsNaN(thumbWidth))
                 {
-                    thumb.Arrange(new Rect(firstWidth, 0, thumbWidth, finalSize.Height));
-                }
-
-                if (increaseButton != null)
-                {
-                    increaseButton.Arrange(new Rect(firstWidth + thumbWidth, 0, remaining - firstWidth, finalSize.Height));
+                    if (thumb != null)
+                    {
+                        thumb.Arrange(new Rect(firstWidth, 0, thumbWidth, finalSize.Height));
+                    }
+
+                    if (increaseButton != null)
+                    {
+                        increaseButton.Arrange(new Rect(firstWidth + thumbWidth, 0, remaining - firstWidth, finalSize.Height));
+                    }
                 }
             }
             else
@@ -158,14 +161,17 @@ namespace Avalonia.Controls.Primitives
                     decreaseButton.Arrange(new Rect(0, 0, finalSize.Width, firstHeight));
                 }
 
-                if (thumb != null)
-                {
-                    thumb.Arrange(new Rect(0, firstHeight, finalSize.Width, thumbHeight));
-                }
-
-                if (increaseButton != null)
+                if (!double.IsNaN(thumbHeight))
                 {
-                    increaseButton.Arrange(new Rect(0, firstHeight + thumbHeight, finalSize.Width, Math.Max(remaining - firstHeight, 0)));
+                    if (thumb != null)
+                    {
+                        thumb.Arrange(new Rect(0, firstHeight, finalSize.Width, thumbHeight));
+                    }
+
+                    if (increaseButton != null)
+                    {
+                        increaseButton.Arrange(new Rect(0, firstHeight + thumbHeight, finalSize.Width, Math.Max(remaining - firstHeight, 0)));
+                    }
                 }
             }