mm macros put footers off end of page if using larger type

[Jim Avera]

I don't know if anybody is supporting the mm macros, but there
is a bug which lands page footers too far down the page (or entirely
off the bottom) if you try to make a "large type" edition of a document.

#!/bin/sh
groff -mm -Tps <<'EOF' | gv -  # bottom footer is mostly invisible
.\" N.B. Pgps=1
.S 20
.PH "'three'part'header'"
.PF "'three'part'footer'"
.P
blah blah blah blah blah blah blah blah blah blah blah
blah blah blah blah blah blah blah blah blah blah blah
blah blah blah blah blah blah blah blah blah blah blah
blah blah blah blah blah blah blah blah blah blah blah
blah blah blah blah blah blah blah blah blah blah blah
blah blah blah blah blah blah blah blah blah blah blah
blah blah blah blah blah blah blah blah blah blah blah
blah blah blah blah blah blah blah blah blah blah blah
blah blah blah blah blah blah blah blah blah blah blah
blah blah blah blah blah blah blah blah blah blah blah
blah blah blah blah blah blah blah blah blah blah blah
blah blah blah blah blah blah blah blah blah blah blah
blah blah blah blah blah blah blah blah blah blah blah
.P
EOF