Clint N. Suhks has the solution, as he/she stated it above.
Thanks!
Just follow his solution.
I went to "Account", "My Preferences", then set both Thread pagination and Browsing to 20 messages, as recommended by CNS, and indeed, the sidebar came back, as well as the full page is showing and you can go to the next page.
Merry Christmas, All!
That didn't work for me.
Actually I don't miss the sidebar as it has a lot of outdated material on it. -tom