message: limit the length of history to 30.
diff: @@ -47,6 +47,9 @@ class ViewerHistory {
  this.last().temporary = false
  }
  this._history.push({scroll: scroll, temporary: false})
  + if (this.length() > 30) {
  + this._history = this._history.slice(this.length() - 30)
  + }
  this._current = this.lastIndex()
  }
  }
  