Thank you all for your comments and suggestions. It can be a frustrating process, but better decisions come out in the end.
I have submitted the code to Kovid. My decisions on the questions were:
1. Support two- and three-value boolean columns via a tweak. I ended up setting the default to
three-value.
2. Tag browser categories are displayed even if the underlying columns are hidden.
3. Restrictions are indicated by all of:
- The restriction name is shown in the set restriction box. Note that this box is associated with the tags browser, so it will be hidden when the tag browser is hidden.
- The 'number of matches' box is changed to display '(n of m)', where n is the number of books matching a search, and m is the number of books matching the restriction. When there is no search, n and m are the same. When there is no restriction, the box displays '(n of all)'
- The 'number of matches' box is colored yellow when a restriction is in force.
See
here for a somewhat more detailed discussion of what changes are included in the submission.
Note that Kovid might change any of these decisions or decide not to provide some of the underlying features.