How to make div on top of all other control

Since you want to cover the whole screen, I recommend this:

#overlayDiv {
    position: fixed;
    top: 0;
    left: 0;
    width: 100%;
    height: 100%;
    z-index:99;
}

Note, you don't have to set the display and visibility properties. Also, don't set padding or margin on this element! If you want it to have a padding, set a margin on its child/children.

Also, make sure that the DIV in question is a direct child of the BODY element.