var contentHeight = 600;
var contentWidth = 800;

var subMenuTopPos = new Array();
var subMenuLeftPos = new Array();
	  
function openSubmenu( id ) {
  var subMenu = document.getElementById( id );
  // calculate new top position
  /*
  var topPos = parseInt( subMenu.style.top, 10);
  if( subMenuTopPos[id] == null ) {
    subMenuTopPos[id] = topPos;
  }
  var innerHeight = document.body.offsetHeight;
  if( innerHeight > contentHeight ) {
    topPos = subMenuTopPos[id] + (innerHeight-contentHeight)/2;
  } else {
    topPos = subMenuTopPos[id];
  }
  subMenu.style.top = topPos + "px";
  */
  if( navigator.appVersion.indexOf("MSIE") == -1 ) {
    // calculate new left position
    var leftPos = parseInt( subMenu.style.left, 10);
    if( subMenuLeftPos[id] == null ) {
      subMenuLeftPos[id] = leftPos;
    }
    var innerWidth = document.body.offsetWidth;
    if( innerWidth > contentWidth ) {
      leftPos = subMenuLeftPos[id] + (innerWidth-contentWidth)/2;
    } else {
      leftPos = subMenuLeftPos[id];
    }
    subMenu.style.left = leftPos + "px";
  }
  subMenu.style.visibility = "visible";
}
function closeSubmenu( id ) {
  document.getElementById( id ).style.visibility = "hidden";
}