

I fixed this issue. I was trying different themes. One theme had added a tiddler
$:/config/SideBarSegments/Visibility/$:/core/ui/SideBarSegments/page-controls
Removing this tiddler fixed the issue.
Thank you Tones for sharing your knowledge. These were new to me.
Until you showed me I did not know these SideBarSegments visibility tiddlers existed.
Right. Surprisingly, docs did not have anything on this special config title. It is mentioned at only one place.
https://tiddlywiki.com/#Page%20and%20tiddler%20layout%20customisation