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Current result:

Expected result:

Hope it will not beat optimization too much

The main reason subpanels don't have functionality for this is twofold. For one, it'd be difficult to expose a convenient way to configure the behavior, and secondly, subpanels being present or not actually change the size of the panel, and would affect placement calculations, and may require more complex behavior to handle.

Maybe other possible solution might be just adjusting height of the sidebar element, when subpanel isn't empty?