the problem is caused by the MainDiv z-index setting.
change the z-index:1:
#MainDiv {
position:relative;
width:979px;
min-height: 100%;
height: auto !important;
height: 100%;
z-index:1;
background-color: #FFF;
margin-right: auto;
margin-left: auto;
border-top-width: 0px;
border-right-width: 1px;
border-bottom-width: 0px;
border-left-width: 1px;
border-top-style: none;
border-right-style: solid;
border-bottom-style: none;
border-left-style: solid;
border-right-color:#000;
border-left-color: #000;
top: 0px;
margin-top: 0px;
margin-bottom: 0px
}
to -1:
#MainDiv {
position:relative;
width:979px;
min-height: 100%;
height: auto !important;
height: 100%;
z-index:-1;
background-color: #FFF;
margin-right: auto;
margin-left: auto;
border-top-width: 0px;
border-right-width: 1px;
border-bottom-width: 0px;
border-left-width: 1px;
border-top-style: none;
border-right-style: solid;
border-bottom-style: none;
border-left-style: solid;
border-right-color:#000;
border-left-color: #000;
top: 0px;
margin-top: 0px;
margin-bottom: 0px
}