Currently, we need to set the Height of the container to a very small value to collapse it to its minimum size when some children aren't visible.
This need to be made in code behind, or via binding, and we need to do it for each container which can have children (direct or indirect) hidden. Not developers friendly at all.
And this only work if the contain itself is always visible, otherwise no correct collapsing will be made (Telerik told me that "The space taken up by a panel item will be collapsed when the panel is invisible based on the panel's initial height.")

A container should always be collapsed to the height taken by its visible children. When someone want to have an empty space, he should use padding, not the height of an invisible item.