I'm developing an UI which asks the user for confirmation if he really wants to close the window if there's still a running process. According to the GTK documentation returning true in the delete-event handler should do the job but it doesn't work for me as the window closes nevertheless.

I reduced it to a minimal working example which produces the same problem.

void main(string[] args)
    import std.stdio;
    import gdk.Event;
    import gtk.Main;
    import gtk.MainWindow;
    import gtk.Widget;


    MainWindow window = new MainWindow("Test of window close button");
    window.setDefaultSize(200, 200);
    window.addOnDelete(delegate bool (Event, Widget) {
        writeln("Close button called");
        return true; // This should prevent the window from closing, but it doesn't


I'm using GtkD 3.8.5. It would be very nice if someone could help me with this. Many thanks :)