code: @Nonnull
    protected final IMPLTYPE internalMarkAsPrepared (@Nonnull final SizeSpec aPreparedSize)
    {
      // Prepare only once!
      internalCheckNotPrepared ();
  
      _setPreparedSize (aPreparedSize);
      return thisAsT ();
    }
docs: INTERNAL method. Do not call from outside!
  
  @param aPreparedSize
  The new prepared size without margin, border and padding.
  @return this