I noticed with the change from man to man-db that I
could no longer drill down into man pages using ctrl-[ in vim
(I use vim as my man page viewer for colors and drill down).

The basic cause of this is that man is filtered through
$MANPAGER even if output not a terminal. This is demonstrated with:

    MANPAGER="tr a-z ." man man | cat

Therefore if you set MANPAGER to be vim, the following gives a vim warning

    man man | cat

vim also does `/usr/bin/man | col -b` internally when ctrl-[ done on a reference. That in turn will fail with the same error.

This also breaks info -> man fallback. info seems to do something like
`man word | less` and that will break if MANPAGER is being set to vim for
eg. which is confirmed with:

   MANPAGER=should_not_run info selinux

I can work around this for vim at least by doing this in my .vimrc

  augroup man
    au!
    "Ensure vim is not recursively invoked (man-db does this)
    "when doing ctrl-[ on a man page reference
    au FileType man let $MANPAGER=""
  augroup END

However, I don't think I should have to do this.
Also I'm not sure anyone would depend on $MANPAGER
always being used, given it wasn't for the previous "man" package.