another hack to solve the problem created by previous commit c4e3518

Use like this:
new modifier(
  function (event) {
    if (event.faked_by_overlay)
      return false;
    return event.isMeta || isMeta;
  },
  function (event) { event.isMeta = true; });

Still, another problem is meta key being released when another app is
focused (e.g. meta-<tab> is used for app-switching)