Fixed stack panel children spacing.

src/Avalonia.Controls/StackPanel.cs

@@ -233,6 +233,11 @@ namespace Avalonia.Controls
 
             foreach (Control child in Children)
             {
+                if (!child.IsVisible)
+                {
+                    continue;
+                }
+
                 double childWidth = child.DesiredSize.Width;
                 double childHeight = child.DesiredSize.Height;