For example, this helps some forms look visually better when I'm mixing various UI controls, because a combobox and a spinner don't always havethe same default height.
It sounds potentially interesting, but also potentially complicated. If this is the primary use case, I think it may already be addressed with TablePane or BoxPane: TablePane always gives all cells in a row the max. height of the cells, and BoxPane with fill:true does the same.
