code: function findViewForLine(cm, lineN) {
      if (lineN >= cm.display.viewFrom && lineN < cm.display.viewTo)
        return cm.display.view[findViewIndex(cm, lineN)];
      var ext = cm.display.externalMeasured;
      if (ext && lineN >= ext.lineN && lineN < ext.lineN + ext.size)
        return ext;
    }
docs: Find a line view that corresponds to the given line number.