class CustomPanel : Panel
{
    public double MinItemWidth
    {
        get { return (double)GetValue(MinItemWidthProperty); }
        set { SetValue(MinItemWidthProperty, value); }
    }

    public static readonly DependencyProperty MinItemWidthProperty =
        DependencyProperty.Register(nameof(MinItemWidth), typeof(double), typeof(CustomPanel), new PropertyMetadata(100.0, (d, e) => (d as CustomPanel)?.InvalidateMeasure()));

    protected override Size MeasureOverride(Size availableSize)
    {
        if (Children.Count > 0)
        {
            var numberOfChildrenPerRow = Math.Floor(availableSize.Width / MinItemWidth);
            var widthPerChild = availableSize.Width / numberOfChildrenPerRow;

            foreach (UIElement child in Children)
            {
                child.Measure(new Size(widthPerChild, availableSize.Height));
                availableSize.Height = Math.Max(availableSize.Height, child.DesiredSize.Height);
            }
        }

        if (Double.IsPositiveInfinity(availableSize.Height) || Double.IsPositiveInfinity(availableSize.Width))
        {
            return new Size(0, 0);
        }

        return availableSize;
    }

    protected override Size ArrangeOverride(Size finalSize)
    {
        double heightOfPreviousRows = 0;
        double heightOfCurrentRow = 0;

        if (Children.Count > 0)
        {
            var numberOfChildrenPerRow = (int)Math.Floor(finalSize.Width / MinItemWidth);
            var widthPerChild = finalSize.Width / numberOfChildrenPerRow;

            int i = 0;

            foreach (UIElement child in Children)
            {
                if (i % numberOfChildrenPerRow == 0)
                {
                    heightOfPreviousRows += heightOfCurrentRow;
                    heightOfCurrentRow = 0;
                }

                heightOfCurrentRow = Math.Max(heightOfCurrentRow, child.DesiredSize.Height);

                var x = (i % numberOfChildrenPerRow) * widthPerChild;
                var y = heightOfPreviousRows;

                child.Arrange(new Rect(new Point(x, y), new Size(widthPerChild, child.DesiredSize.Height)));

                ++i;
            }
        }

        return finalSize;
    }
}