Thank you for writing.
To do that, once you expand the first panel, you need to check its size and according to its new size, set the Location of the rest of the panels i.e if you have three panels, all collapsed with height 40 px. The first is located at 0,40, the second at 0,80, the third at 0,120. Once you expand the first, check its size (let's say 140), deduct the initial panel size (140 - 40) and add the difference to the Y location of the other panels i.e. second panel location should be 0,140 and third panel location should be 0,180.
I hope that you find this information useful.
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