Should vertical motions be in vees or ems? Where does the baseline go?

[G. Branden Robinson]

In Savannah #61437[1], Bjarni asserted that our groff(7) page was wrong
to say that the \d and \r escape sequence move in vees (the
documentation of the \u escape sequence did not draw his comment and the
default scaling unit of \v [vees], did not draw his comment).

After a bit of back and forth, I found myself surprised by groff's
behavior.  It seemed Bjarni was right, and we're moving in ems after
all, and a commit I made in May claiming errata in CSTR#54 was
wrong was therefore in error[2].

But our source code does indeed request nodes of the "vmotion"
type[3][4], and when I tried to turn to Heirloom troff to try to find
support for one practice or the other, I was dismayed to find a third,
possibly.

My attempt to illuminate the issue by drawing a baseline rule also
illuminated a difference between Heirloom and groff regarding where the
baseline is.

Here's my input document.

.sp 2v
\l'\n(.lu'\h'|0'
\" change the G to H for Heirloom
Gfoo\dbar\rbaz\dqux
foo\dbar\u\ubaz\dqux
foo\v'0.5v'bar\v'-1v'baz\dqux
foo\v'0.5m'bar\v'-1m'baz\dqux

So I thought I would put a couple of images before the real typographers
on this list and ask: what are the bugs in the respective
implementations?  What would an ideal model of output from the above
input look like?

Regards,
Branden

[1] https://savannah.gnu.org/bugs/?61437
[2] be0bd23b1b9170ca31dec4e151a65cf2c333fe6b
[3] 
https://git.savannah.gnu.org/cgit/groff.git/tree/src/roff/troff/input.cpp#n2029
[4] 
https://git.savannah.gnu.org/cgit/groff.git/tree/src/roff/troff/input.cpp#n2159

vert-groff.png
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vert-heirloom.png
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