var spread = new GC.Spread.Sheets.Workbook($("#ss")[0]);
var activeSheet = spread.getActiveSheet();
activeSheet.setRowCount(4, GC.Spread.Sheets.SheetArea.colHeader);

$("#ss").click(function (e) {
    // 列ヘッダセル内で、マウスでクリックされたセルのインデックスを取得します。
    var offset = $("#ss").offset();
    var x = e.pageX - offset.left;
    var y = e.pageY - offset.top;
    var target =spread.getActiveSheet().hitTest(x, y);

    if(target &&
        target.rowViewportIndex === -1 &&
        (target.colViewportIndex === 0 || target.colViewportIndex === 1)){
        console.log("Row index of mouse-clicked column header cells: " + target.row);
        console.log("Column index of mouse-clicked column header cells: " + target.col);
    }
});