FrameBox heights wrong when turning Xwt-Gtk3 widgets to Gtk3

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using System;
using Xwt;

class xwt3test
{
	[STAThread]
	static void Main () {
		Application.Initialize("Xwt.GtkBackend.GtkEngine, Xwt.Gtk3, Version=1.0.0.0");
        var mainWindow = new Gtk.Window("Xwt/Gtk Demo Application")
        {
            Title = "Xwt Demo Application",
            WidthRequest = 500,
            HeightRequest = 400
        };
		Gtk.VBox vbox1 = new Gtk.VBox();
		Xwt.VBox infoBox = new Xwt.VBox()
        {
            CanGetFocus = false
        };
        Xwt.FrameBox mbox = new Xwt.FrameBox(infoBox)
        {
            CanGetFocus = false
        };
		vbox1.PackStart((Gtk.Widget)Xwt.Toolkit.CurrentEngine.GetNativeWidget(mbox), false, false, 0);
		infoBox.PackStart(new Label("test1"));
		infoBox.PackStart(new Label("test2"));
		infoBox.PackStart(new Label("test3"));
		infoBox.PackStart(new Label("test4"));
		infoBox.PackStart(new Label("test5"));
		mainWindow.Add(vbox1);
        mainWindow.ShowAll();
        Application.Run();
        mainWindow.Dispose();
	}
}

Results in:

A pure Xwt approach is fine, i.e.

using System;
using Xwt;

class xwt3test
{
	[STAThread]
	static void Main () {
		Application.Initialize("Xwt.GtkBackend.GtkEngine, Xwt.Gtk3, Version=1.0.0.0");
        var mainWindow = new Window()
        {
            Title = "Xwt Demo Application",
            Width = 500,
            Height = 400
        };
		Xwt.VBox infoBox = new Xwt.VBox()
        {
            CanGetFocus = false
        };
        Xwt.FrameBox mbox = new Xwt.FrameBox(infoBox)
        {
            CanGetFocus = false
        };
		infoBox.PackStart(new Label("test1"));
		infoBox.PackStart(new Label("test2"));
		infoBox.PackStart(new Label("test3"));
		infoBox.PackStart(new Label("test4"));
		infoBox.PackStart(new Label("test5"));
		mainWindow.Content = mbox;
        mainWindow.Show();
        Application.Run();
        mainWindow.Dispose();
	}
}

leads to

This is why the text in Help/About in MonoDevelop-Gtk3 is broken.

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It's the Xwt.FrameBox - putting the Xwt.VBox directly into the Gtk.VBox is fine, putting the Xwt.FrameBox containing the Xwt.VBox into the Gtk.VBox causes the compression issue above.