Opera javascript

Question

This code function is runing when mouseover in div with interval 10ms

It is using framework prototype 1.6.0.2

In all another browsers it works fine but not in opera

function openMenu(){
    var pleft = $("slidemenubar2").getStyle("left");

    if(parseInt(pleft) < 0){
         $("slidemenubar2").setStyle({left: (parseInt(pleft)+10)+'px'});
         alert($("slidemenubar2").getStyle("left"));//always <0
    }
}

How can this be? (Don't understand).