__blockListShowDuringTransition() {
    this.__blockListShow = true;
    // We need this timeout to make sure click handler triggered by keyup (space/enter) of
    // button has been executed.
    // TODO: alternative would be to let the 'checking' party 'release' this boolean
    // Or: call 'stopPropagation' on keyup of keys that have been handled in keydown
    setTimeout(() => {
      this.__blockListShow = false;
    }, 200);
  }
Normally, when textbox gets focus or a char is typed, it opens listbox.
In transition phases (like clicking option) we prevent this.
@private