final ExpandItem expandable = new ExpandItem(bar, SWT.NONE, 0);
expandable.setControl(composite);  			

Sash sash = new Sash(composite, SWT.HORIZONTAL);
sash.setLayoutData(new GridData(SWT.FILL, SWT.NONE, true, false));
sash.addMouseListener(new MouseAdapter() {
    @Override
    public void mouseUp(MouseEvent e) {
       expandable.setHeight(expandable.getHeight() + e.y);
    }
});