//resizes a div to fill out the height of the window
//the offset is added, so negative offsets are used to subtract
function resizeDivHeight(divID, offset, minimum) {
    newHeight = window.innerHeight + offset;
    if(newHeight < minimum) {
        newHeight = minimum;
    }
    document.getElementById(divID).style.height = newHeight + "px";
    //call the resize function when the window size changes
    window.onresize = function() {
        newHeight = window.innerHeight + offset;
        if(newHeight < minimum) {
            newHeight = minimum;
        }
        document.getElementById(divID).style.height = newHeight + "px";
        }
    }