Ensure VirtualizingStackPanel correct element size estimate and fix #17460 (#17461)

Co-authored-by: Emmanuel Hansen <[email protected]>

Author: BobLd

src/Avalonia.Controls/VirtualizingStackPanel.cs

@@ -2,7 +2,6 @@
 using System.Collections.Generic;
 using System.Collections.Specialized;
 using System.Diagnostics;
-using System.Drawing;
 using System.Linq;
 using Avalonia.Controls.Primitives;
 using Avalonia.Controls.Utils;
@@ -164,6 +163,9 @@ protected override Size MeasureOverride(Size availableSize)
                 _realizedElements ??= new();
                 _measureElements ??= new();
 
+                // We need to set the lastEstimatedElementSizeU before calling CalculateDesiredSize()
+                _ = EstimateElementSizeU();
+
                 // We handle horizontal and vertical layouts here so X and Y are abstracted to:
                 // - Horizontal layouts: U = horizontal, V = vertical
                 // - Vertical layouts: U = vertical, V = horizontal