How to make Lightbox scroll but not background

Question

I am trying to create a lightbox to show some custom data. There are two problems I am facing which I do not know how to handle.

1. First problem is I don't want the page to scroll(vertically) when lightbox is visible.

2. Second problem is I want the lightbox to scroll if the content is bigger than the viewport of the browser.

But I am not sure how to approach this. I tried overflow-x:scroll; overflow-y:scroll; overflow-x:scroll; overflow-y:scroll; but it shows the scrollbar but doesn't scroll.

Following is the code I currently have :

Html:

<div class="lightbox">
    <section class="image-app">
        <div class="app-bar">
            <div class="app-close">
                <span class="glyphicon glyphicon-remove" aria-hidden="true"></span>
            </div>
        </div>
        <div class="container_fluid">
            <div class="row">
                <div class="col-md-8">
                    <div class="app-image">

                    </div>
                </div>
                <div class="col-md-4">
                    <div class="side-bar">

                    </div>
                </div>
            </div>
        </div>
    </section>
</div>

CSS:

.lightbox {
    display: none;
    position: fixed;
    background: rgba( 0, 0, 0, 0.7 );
    bottom: 0;
    right: 0;
    top: 0;
    left: 0;
    z-index: 1000;
}
.image-app {
    display: none;
    position: relative;
    background: #fafafa;
    top: 3%;
    left: 3%;
    width: 94%;
    min-height: 94%;
    overflow-x:scroll;
    overflow-y:scroll;
    border: #8e8e8e;
    z-index: 1001;
    padding: 10px;

    -webkit-box-shadow: 0px 0px 21px 0px rgba(0,0,0,1);
    -moz-box-shadow: 0px 0px 21px 0px rgba(0,0,0,1);
    box-shadow: 0px 0px 21px 0px rgba(0,0,0,1);
}
.app-close {
    padding: 10px 10px;
    text-align: right;
    font-size: 150%;
    cursor: pointer;
}
.app-image {
    text-align: center;
    background: #ffffff;
}
.side-bar {
    position: relative;
    height: 100%;
    width: 100%;
}

Javascript :

    $ ( '.item' ).click( function ( event ) {

    event.preventDefault (  );
    event.stopPropagation (  );



    $( '.lightbox' ).show (  );
    $( '.image-app' ).slideToggle ( "fast" );

} );

Answer 1

if ($('.lightbox').css('display') != 'none') {
    $('body').on({'mousewheel': function(e) {
        e.preventDefault();
        e.stopPropagation();
    }
})
}

var div = $(".lightbox").height();
var win = $(window).height();

if (div > win ) {
    $(".lightbox").css("overflow-x","scroll");
}