I suppose you are aiming at something like this:
This is a demo I made for a different case and demonstrates how the PanelBar has a fixed height and the expanded item(s) always take up as much space as possible, with a fixed ratio 75% / 25%. In case you have more items, or the ratio should vary, the implementation will become more complex.
Using 100% heights will not lead to the desired result, because you actually need "all remaining available space", which is a different thing. Moreover, according to web standards, a percentage height requires an explicit height set to the parent element, which you probably don't have. That's why height calculations are used in the above demo, and heights are set in pixels.
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