Gtk.TextView.prototype.set_border_window_size

function set_border_window_size(type:Gtk.TextWindowType, size:Number):void {
    // Gjs wrapper for gtk_text_view_set_border_window_size()
}

Sets the width of Gtk.TextWindowType.left or Gtk.TextWindowType.right, or the height of Gtk.TextWindowType.top or Gtk.TextWindowType.bottom. Automatically destroys the corresponding window if the size is set to 0, and creates the window if the size is set to non-zero. This function can only be used for the "border windows," it doesn't work with #GTK_TEXT_WINDOW_WIDGET, #GTK_TEXT_WINDOW_TEXT, or #GTK_TEXT_WINDOW_PRIVATE.