In eel/eel/eel-gdk-extensions.c:

void
eel_gdk_window_focus (GdkWindow *window, guint32 timestamp)
{
        gdk_error_trap_push ();
        XSetInputFocus (GDK_DISPLAY (),
                        GDK_WINDOW_XWINDOW (window),
                        RevertToNone,
                        timestamp);
        gdk_flush();
        gdk_error_trap_pop ();
}

This is a no-no, it causes keyboard navigation to break (i.e. you get into
situations where keynav of the desktop fails since no window has focus).

It's an accessibility/keynav shop-stopper.