gistfile1.js

function getSquare(canvas, evt) {
    var rect = canvas.getBoundingClientRect();
    var values = this.scroller.getValues();
    return {
        x: Math.ceil((((evt.clientY - rect.top + values.top) / values.zoom) - ((evt.clientY - rect.top + values.top)/values.zoom) % cellHeight)  / cellHeight),
        y: Math.ceil((((evt.clientX - rect.left + values.left) / values.zoom) - ((evt.clientX - rect.left + values.left)/values.zoom) % cellWidth)  / cellWidth)
    };
}