Horizontal Layout and ExpandRatio with Unit.EM

Hi all,

I have a HorizontalLayout with several Components inside.
I add all Components to the Layout and set 1 Component with the ExpandRation 1. This is the only Component that should expand.
There is also a Component with the With 20em.
The rest has sizeUndefined.

In my understanding EM is a relative Size to the Font, so it is kinda like a fixe size, so why does the Component with the em-with disapear?
It is rendered if I use the Unit.Pixels there.

thanks for any help.