Gtk.Editable.prototype.get_selection_bounds

function get_selection_bounds():Boolean {
    // Gjs wrapper for gtk_editable_get_selection_bounds()
}

Retrieves the selection bound of the editable. start_pos will be filled with the start of the selection and end_pos with end. If no text was selected both will be identical and false will be returned.

Note that positions are specified in characters, not bytes.