I re-opened that drawing, and of course, the tabs were still very small.
I deleted numerous paper spaces until there were only a couple left.
Closed and saved with a different name, then re-opened.
The working space tabs are still extremely small.
I have used the basis of this drawing as a template for quite some time, over a fair number of TC versions.
This symptom is a surprise
I have re-created the drawing and, so far, seems OK. Just a pain when it happens with an existing drawing / project.