Simple demonstration:
http://zkfiddle.org/sample/jgobe5/5-HBox-resize-issue

Description:
1. Click to button.
2. Grid C is shown.
3. Height of content of inner window is bigger than height of window. Therefore vertical scrollbar is shown.
4. Grid C has correct width but a HBox has an old width and the result is that a horizontal scrollbar is shown. That is incorrect.