You'd be surprised how flexible some of the GUI toolkits on the Amiga actually were. MUI is based on the concept that layout is dynamic and flexible.

It would dynamically calculate a minimum window size based on the components inside and you could then resize the window as you wanted with things calculating and resizing as you go, adapting to font selections and themes.

The actual implementation is archaic by todays standards but it did use attributes on UI objects to determine min/max sizing and other things, similar to CSS in concept.