None of the layout Panes can correctly compute the layout for combined HORIZONTAL and VERTICAL bias.
After discussion with Richard and Steve, we decided to give HORIZONTAL bias higher priority, so the VERTICAL bias will not work in such Panes unless all of the children are either unbiased or biased vertically.

Some of our Panes doesn't support vertical bias at all, like HBox. This should be evaluated and decide whether it's OK to restrict the Panes to only Horizontal bias (if so for HBox, should VBox be vertical bias only?).

Javadoc needs to updated as well for all layout Panes.