SplitPanel.cs

public class SplitPanel : Panel
{
    private double[] positions = Array.Empty<double>();

    protected override Size MeasureOverride(Size availableSize)
    {
        if (this.positions.Length < this.InternalChildren.Count)
        {
            this.positions = new double[this.InternalChildren.Count];
        }

        var sum = 0.0;
        var width = 0.0;
        for (var i = 0; i < this.InternalChildren.Count; i++)
        {
            var child = this.InternalChildren[i];
            child.Measure(availableSize);
            sum += child.DesiredSize.Height;
            width = Math.Max(width, child.DesiredSize.Width);
        }

        this.positions[0] = 0;
        if (sum <= availableSize.Height)
        {
            for (var i = 0; i < this.InternalChildren.Count - 1; i++)
            {
                this.positions[i + 1] = this.positions[i] + this.InternalChildren[i].DesiredSize.Height;
            }

            return new Size(
                Math.Min(availableSize.Width, width),
                sum);
        }

        var maxHeight = MaxHeight();
        for (var i = 0; i < this.InternalChildren.Count; i++)
        {
            var child = this.InternalChildren[i];
            if (child.DesiredSize.Height > maxHeight)
            {
                child.Measure(new Size(availableSize.Width, maxHeight));
            }

            if (i < this.InternalChildren.Count - 1)
            {
                this.positions[i + 1] = this.positions[i] + Math.Min(this.InternalChildren[i].DesiredSize.Height, maxHeight);
            }
        }

        return new Size(
            Math.Min(availableSize.Width, width),
            availableSize.Height);

        double MaxHeight()
        {
            var children = ListCache.Borrow(this.InternalChildren);
            var remaining = availableSize.Height;
            while (FindSmall() is { } small)
            {
                children.Remove(small);
                remaining -= small.DesiredSize.Height;
            }

            return remaining / ListCache.Return(children);

            UIElement? FindSmall()
            {
                if (children.Count < 2)
                {
                    return null;
                }

                foreach (var child in children)
                {
                    if (child.DesiredSize.Height < remaining / children.Count)
                    {
                        return child;
                    }
                }

                return null;
            }
        }
    }

    protected override Size ArrangeOverride(Size finalSize)
    {
        for (var i = 0; i < this.InternalChildren.Count; i++)
        {
            this.InternalChildren[i].Arrange(
                new Rect(
                    0,
                    this.positions[i],
                    finalSize.Width,
                    Height()));

            double Height()
            {
                if (i < this.InternalChildren.Count - 1)
                {
                    return this.positions[i + 1] - this.positions[i];
                }

                return finalSize.Height - this.positions[^1];
            }
        }

        return finalSize;
    }

    private static class ListCache
    {
        private static List<UIElement>? cached = new();

        internal static List<UIElement> Borrow(UIElementCollection elements)
        {
            var list = cached ?? throw new InvalidOperationException("Not returned?");
            cached = null;
            for (var i = 0; i < elements.Count; i++)
            {
                list.Add(elements[i]);
            }

            return list;
        }

        internal static int Return(List<UIElement> list)
        {
            var count = list.Count;
            list.Clear();
            cached = list;
            return count;
        }
    }
}