message: fix ViewerHistory.set
diff: @@ -47,9 +47,9 @@ class ViewerHistory {
  this.last().temporary = false
  }
  this._history.push({scroll: scroll, temporary: false})
  - }
  this._current = this.lastIndex()
  }
  + }
  back() {
  if (this.length() === 0) {
  