What I tried so far:
ScrolledComposite sc = new ScrolledComposite(parent, SWT.V_SCROLL | SWT.H_SCROLL);
final TabFolder tabFolder = new TabFolder(sc, SWT.TOP);
The Javadoc of ScrolledComposite describes the two ways to use it, including example code. To sum them up:
ScrolledCompositeon the control/composite itself
ScrolledCompositethe minimum size to use for its content.
Currently, you're doing neither. You're setting the size on the
ScrolledComposite, but unless you don't use a layout manager, that doesn't make much sense. In any case, see the above link for some official example code.