Using UIBuilder you don't need to create manually a scroller. Check the following script by resizing until vertical or horizontal scroll bars appear automatically.
| dialog morphList row morph1 morph2 morph3 morph4 |
morph1 := BorderedMorph new hResizing: #spaceFill.
morph2 := BorderedMorph new.
morph3 := BorderedMorph new.
morph4 := BorderedMorph new.
row := UITheme builder newRow: {morph1 . morph2 . morph3}.
morphList := UITheme builder
newMorphListFor: (ListModel new list: {row . morph4})
list: #list
getSelected: #selectionIndex
setSelected: #selectionIndex:
help: 'This is a morph list'.
dialog := UITheme builder newWindowFor: nil title: 'titleString'.
dialog addMorph: morphList fullFrame: (LayoutFrame fractions: (0@0 corner: 1@0.5)).
dialog openInWorld