Gtk.TextView.prototype.buffer_to_window_coords

function buffer_to_window_coords(win:Gtk.TextWindowType, buffer_x:Number, buffer_y:Number):void {
    // Gjs wrapper for gtk_text_view_buffer_to_window_coords()
}

Converts coordinate (buffer_x, buffer_y) to coordinates for the window win, and stores the result in (window_x, window_y).

Note that you can't convert coordinates for a nonexisting window (see Gtk.set_border_window_size).