Font-size css specifiers on GTKToggleButton labels cause inaccurate fullscreen rendering

[Domitian (Dominik)]

Using gtk_window_fullscreen_on_monitor() I am able to choose which display to launch the fullscreen application into. However, though I am using monitor 1 (5:4), it appears it is using the measurements or ratio of my other display (16:9), with the end result that it does not go fully full screen, as one can see in the below print screen:

, where a portion of the desktop is still visible underneath the application, and some buttons are cut off at the right end.

Any tips to fix this or is it a bug? It appears to be a problem with the labels, specifically their font-size specifier. They also shape the size of the toggle button instead of the other way around.

The system is Debian bookworm.

[Domitian (Dominik)]

My current button creation, the sprintf label command is commented out as that messes up the fullscreen rendering:

static void make_toggles (GtkWidget * window) {
    GtkWidget * stop_box;
    GtkWidget * box_row;
    short i; 

    stop_box = gtk_box_new(GTK_ORIENTATION_VERTICAL, 10);
    
    //initialise stops on this panel
    char label[32]; label[0] = '\0'; 
    short curr_row = 0;

    box_row = gtk_box_new(GTK_ORIENTATION_HORIZONTAL, 2);
    gtk_box_append(GTK_BOX(stop_box), box_row);
    for (i = 0; i< rnum; i++) {
        if (curr_row != rstops[i].grid_y) {
            box_row = gtk_box_new(GTK_ORIENTATION_HORIZONTAL, 2);
            gtk_box_append(GTK_BOX(stop_box), box_row);
            curr_row += 1;
        }
        //sprintf(label, rstops[i].label);
        sprintf(label, "%d", i);
        stop = gtk_toggle_button_new_with_label(label);
        gtk_widget_set_name(stop, "stop_button");
        gtk_widget_set_margin_start(stop, (rstops[i].margin_left * 4) + 5); 
        g_signal_connect(stop, "toggled", G_CALLBACK(stop_toggle), NULL);
        gtk_box_append(GTK_BOX(box_row), stop);   
    }
    
    gtk_widget_set_name(stop_box, "window_main");
    gtk_window_set_child (GTK_WINDOW (window), stop_box);
}

Update: the issue appears to be the font-size: xx-large property that applies to my buttons. I can remove that variable, but then the font is rather small.
A proper fullscreen once font-size is removed:

I would like to use gtk_label_set_wrap() to help the labels fit inside their buttons, but not sure how to extract a GTKLabel from a GTKToggleButton.

EDIT: I figured it out, just create a label and set is as the child of the GTKButton. My final issue is that the text does not center on multi-line labels.

Final EDIT: the centering can be done with gtk_set_justify().