[Groff] odd interaction between .bl and .(c in -me macros

[Dave Kemper]

While I'm on the subject of .bl's behavior in -me, I'm running across
another curious issue: .bl, which ordinarily works at the top of a page,
does not seem to do so when inside a .(c block.  I am not sure if this
is a bug, or if I'm misunderstanding some subtlety of the interaction
between these two macros.

As a minimal example, the first .bl is ignored when the following is
the first thing on a page:

.(c
.bl 20
center me
.bl 20
.)c
.pp
Here is text.

If I place the first .bl outside the centered block, it works in the
top-of-page instance, but of course, this breaks the general instance,
where the .bl should be part of the block.

This behavior does not appear to be the result of no-space mode being in
effect: putting a .rs either before or after the .(c -- or even in both
places -- does not cause the first .bl inside the block to be honored.
I also tested both with and without Tadziu Hoffmann's recent patch to the
.(c macro (posted here Nov 5) to see if that made a difference; it didn't.