$("#button1").click(function () {  
    //Acquire top row (column) index
    var topRow = activeSheet.getViewportTopRow(1);
    var leftCol = activeSheet.getViewportLeftColumn(1);
    alert("Index of top row being displayed: " + topRow + "\n" +
        "Index of first column being displayed: " + leftCol);
});

//Add button control to page
<input type="button" id="button1" value="button1"/>