Sometimes GTK modal dialogs are not modal — bug or feature?

Question

When I create a custom dialog in GTK ( both, GTK2 or GTK3 ) and set it to be modal, all input to other windows of my application is ignored. This works nearly always, but it fails under certain conditions.

When I add a ScrolledWindow containing a TreeView to my dialog, it still works as supposed. But if I fill the TreeView with entries until the ScrolledWindow starts to display its scroll bars --- the modality is suddenly lost and I can click on my other windows!

Here is the most basic example I was able to set up. It's written in Vala, but you'll get the idea:

class MyDialog: Gtk.Dialog {

    public MyDialog() {
        this.modal = true;

        var data = new Gtk.ListStore(1, typeof(string)); 

        // increase this number -- the dialog is not modal anymore!
        for (int i=0; i<2; ++i) {
            Gtk.TreeIter current;
            data.append(out current);
            data.set(current, 0, "Lorem Ipsum"); 
        }

        var render = new Gtk.CellRendererText();

        var column = new Gtk.TreeViewColumn();
            column.pack_start(render, true);
            column.add_attribute(render, "text", 0);

        var treeview = new Gtk.TreeView.with_model(data);
            treeview.append_column(column);
            treeview.show();

        var scroll = new Gtk.ScrolledWindow(null, null);
            scroll.set_size_request(100, 100);
            scroll.add(treeview);
            scroll.show();

        (this.get_content_area() as Gtk.Box).add(scroll);
    }
}

int main (string[] args) {
    Gtk.init (ref args);

    var window = new Gtk.Window();

    window.set_default_size(350, 170);
    window.destroy.connect(Gtk.main_quit);

    var button = new Gtk.Button.with_label("Click me!");
    button.clicked.connect(() => {
        var dialog = new MyDialog();
        dialog.set_transient_for(window);
        dialog.run();
        dialog.destroy();
    });

    window.add(button);
    window.show_all();

    Gtk.main();
    return 0;
}

Compile it with:

valac --pkg gtk+-3.0 main.vala

Am I missing something? Is this behaviour wanted? Or is it a bug? If so, is there a workaround?

EDIT: I investigated a bit further: The problem disappears when the overlay-scrollbars from Ubuntu are uninstalled. So It's not solved yet, but I know where I have to report this...

Answer 1

Definitely a bug. Post a bug report and/or upgrade your GTK+ lib.