code: public TextBox setMask(Character mask) {
          if(mask != null && TerminalTextUtils.isCharCJK(mask)) {
              throw new IllegalArgumentException("Cannot use a CJK character as a mask");
          }
          this.mask = mask;
          invalidate();
          return this;
      }
docs: Sets the current text mask, meaning the substitute to draw instead of the text inside the {@code TextBox}.
  This is normally used for password input fields so the password isn't shown
  @param mask New text mask or {@code null} if there is no mask
  @return Itself