Can you give me more details about what do you mean under "fill the available space"? If this space is the one under the PanelBar, how do you imagine the PanelBar to look at its initialization? Is there an example you can provide? If we take this example, how would you like the PanelBar items to be sized on component initialization?
Please provide more details about the desired functionality and I will prepare an example of the desired functionality if it is possible to be implemented.