Scroll in modal mode ?

Hello,
Why do I need to add two code line in "lightbox.js" too get my scroll bars works in modal mode ?
The function I change is setStyles, here is my change:

     if (stylesArray[i].indexOf('width:') >= 0) {
        var w = stylesArray[i].replace('width:', '');
        item.width = jQuery.trim(w);
        if (item.id == "modalContainer") $('#modalContainer').css('width',w); // line added...
      }
      else if (stylesArray[i].indexOf('height:') >= 0) {
        var h = stylesArray[i].replace('height:', '');
        item.height = jQuery.trim(h);
        if (item.id == "modalContainer") $('#modalContainer').css('height',h); // line added...
     }

Someone tell me if there is a solution that would avoid changing the code module ?
If i do not add line, you can see that a get on the jpg attached...
Sorry for my bad bad english, merci.