A Xamarin.Forms (2.3.2) hack, that short circuit the layout system.

ShortCircuit.cs

public class PresenterTest : ContentPresenter
{
    BoxView _testView;

    public PresenterTest()
    {
        _testView = new BoxView {
            BackgroundColor = Color.Red,
            WidthRequest = 30,
            HorizontalOptions = LayoutOptions.Fill,
            VerticalOptions = LayoutOptions.Fill
        };

        var children = (ObservableCollection<Element>)((ILayoutController)this).Children;
        children.Add(_testView);

        var rand = new Random();
        Device.StartTimer(TimeSpan.FromMilliseconds(100), () =>
        {
            _testView.WidthRequest = rand.Next(50, 80);
            if (relayouts > 20)
            {
                System.Diagnostics.Debug.WriteLine(relayouts);
                return false;
            }

            if (timer++ < 30)
            {
                System.Diagnostics.Debug.WriteLine(relayouts);
                return false;
            }

            return true;
        });
    }

    int timer = 0;
    int relayouts = 0;

    protected override void LayoutChildren(double x, double y, double width, double height)
    {
        relayouts++;
        LayoutChildIntoBoundingRegion(_testView, new Rectangle(width / 2 - _testView.WidthRequest / 2, 50, _testView.WidthRequest, _testView.WidthRequest));
    }
}