AvaloniaUI's Viewbox

Viewbox.cs

using Avalonia;
using Avalonia.Controls;
using Avalonia.Media;
using static System.Math;

public class Viewbox : Decorator
{
    private IControl Root => Child;

    protected override Size MeasureOverride(Size constraint)
    {
        if (Root == null)
        {
            return constraint;
        }

        var availableSize = new Size(double.PositiveInfinity, double.PositiveInfinity);
        Root.Measure(availableSize);
        var desiredSize = Root.DesiredSize;
        var factor = GetFactorToFillUniformly(constraint, desiredSize);
        return new Size(factor.Width * desiredSize.Width, factor.Height * desiredSize.Height);
    }

    protected override Size ArrangeOverride(Size arrangeSize)
    {
        if (Root == null)
        {
            return arrangeSize;
        }

        var desiredSize = Root.DesiredSize;
        var factor = GetFactorToFillUniformly(arrangeSize, desiredSize);

        Root.RenderTransform = new ScaleTransform(factor.Width, factor.Height);
        Root.Arrange(new Rect(new Point(0, 0), desiredSize));

        return new Size(factor.Width * desiredSize.Width, factor.Height * desiredSize.Height);
    }

    private static Size GetFactorToFillUniformly(Size availableSize, Size contentSize)
    {
        var width = DoubleUtil.IsZero(contentSize.Width) ? 0.0 : availableSize.Width / contentSize.Width;
        var height = DoubleUtil.IsZero(contentSize.Height) ? 0.0 : availableSize.Height / contentSize.Height;

        var min = Min(width, height);
        return new Size(min, min);
    }
}

internal class DoubleUtil
{
    const double Tolerance = 0.1D;

    public static bool IsZero(double contentSizeWidth)
    {
        return Abs(contentSizeWidth) < Tolerance;
    }
}