I am afraid that is still the 'Thousand Dollar Question' that we are all waiting for MS to answer and hopefully resolve.
Many had hoped across the various Posts associated with this issue from around the world, that MS were getting this sorted but anecdotal reports suggest that our hopes may have been a little premature, where the return of the Scroll bar was either short lived, not stable or varied acording to which Browser you are using.
The latest thoughts seem to be that you may be able temporarily work around the Issue by refreshing the Contacts / People page using CONTROL (CTRL) F5 since you will probably find that using the Cursor down only displays a limited number of Contacts (eg only around the top 20 of my 165 Contacts) as these have not been populated on the screen beyond the 20th or so entry. However they are still there - you just can't see them and can in most instances be called up and viewed individually by using the Search Field.
Finally be careful if editing a Contact as the details displayed don't always update when amending Contacts and seem to only display the revised / updated content by closing & reopening the OWA or using CTRL F5 to Refresh before re-searching to view the amended Contact.