website/playground/panels.js/createOverlay
function createOverlay(start, end) {
  return {
    token(stream) {
      const { line } = stream.lineOracle;

      if (line < start.line || line > end.line) {
        stream.skipToEnd();
      } else if (line === start.line && stream.pos < start.pos) {
        stream.pos = start.pos;
      } else if (line === end.line) {
        if (stream.pos < end.pos) {
          stream.pos = end.pos;
          return "searching";
        }
        stream.skipToEnd();
      } else {
        stream.skipToEnd();
        return "searching";
      }
    },
  };
}