I just switched to 20 threads per page and the sidebar came back.
Fix it at “Latest Articles: Brevity/Headers/Text”, in top center of page.
Clint N. Suhks has the solution, as he/she stated it above.
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!