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 cache = cm.display.measureLineCache;
    for (var i = 0; i < cache.length; ++i) {
      var memo = cache[i];
      if (memo.text == line.text && memo.markedSpans == line.markedSpans &&
          cm.display.scroller.clientWidth == memo.width &&
          memo.classes == line.textClass + "|" + line.wrapClass)
        return memo;
    }
  }
  function clearCachedMeasurement(cm, line) {
    var exists = findCachedMeasurement(cm, line);
    if (exists) exists.text = exists.measure = exists.markedSpans = null;
  }
  function measureLine(cm, line) {
    
    var cached = findCachedMeasurement(cm, line);
    if (cached) return cached.measure;
    
    var measure = measureLineInner(cm, line);
    var cache = cm.display.measureLineCache;
    var memo = {text: line.text, width: cm.display.scroller.clientWidth,
                markedSpans: line.markedSpans, measure: measure,
                classes: line.textClass + "|" + line.wrapClass};
    if (cache.length == 16) cache[++cm.display.measureLineCachePos % 16] = memo;
    else cache.push(memo);
    return measure;
  }
  function measureLineInner(cm, line) {