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.

if (display.showingTo > to && display.showingTo - to < 20) to = Math.min(end, display.showingTo);
    if (sawCollapsedSpans) {
      from = lineNo(visualLine(doc, getLine(doc, from)));
      while (to < end && lineIsHidden(doc, getLine(doc, to))) ++to;
    }
    
    
    var intact = [{from: Math.max(display.showingFrom, doc.first),
                   to: Math.min(display.showingTo, end)}];
    if (intact[0].from >= intact[0].to) intact = [];
    else intact = computeIntact(intact, changes);
    
    
    
    if (sawCollapsedSpans)
      for (var i = 0; i < intact.length; ++i) {
        var range = intact[i], merged;
        while (merged = collapsedSpanAtEnd(getLine(doc, range.to - 1))) {
          var newTo = merged.find().from.line;
          if (newTo > range.from) range.to = newTo;
          else { intact.splice(i--, 1); break; }
        }
      }
    
    var intactLines = 0;
    for (var i = 0; i < intact.length; ++i) {
      var range = intact[i];
      if (range.from < from) range.from = from;
      if (range.to > to) range.to = to;
      if (range.from >= range.to) intact.splice(i--, 1);