Right now we use codemirror's JavaScript mode on the website but there's a JSX mode now so we should start using that instead.

var innerOff = y - heightAtLine(cm, lineObj);
    var wrongLine = false, adjust = 2 * cm.display.wrapper.clientWidth;
    var measurement = measureLine(cm, lineObj);
    function getX(ch) {
      var sp = cursorCoords(cm, Pos(lineNo, ch), "line",
                            lineObj, measurement);
      wrongLine = true;
      if (innerOff > sp.bottom) return sp.left - adjust;
      else if (innerOff < sp.top) return sp.left + adjust;
      else wrongLine = false;
      return sp.left;
    }
    var bidi = getOrder(lineObj), dist = lineObj.text.length;
    var from = lineLeft(lineObj), to = lineRight(lineObj);
    var fromX = getX(from), fromOutside = wrongLine, toX = getX(to), toOutside = wrongLine;
    if (x > toX) return PosWithInfo(lineNo, to, toOutside, 1);
    
    for (;;) {
      if (bidi ? to == from || to == moveVisually(lineObj, from, 1) : to - from <= 1) {
        var ch = x < fromX || x - fromX <= toX - x ? from : to;
        var xDiff = x - (ch == from ? fromX : toX);
        while (isExtendingChar.test(lineObj.text.charAt(ch))) ++ch;
        var pos = PosWithInfo(lineNo, ch, ch == from ? fromOutside : toOutside,
                              xDiff < 0 ? -1 : xDiff ? 1 : 0);
        return pos;
      }