> Examples?  Are those per chance the cases where Emacs writes only part
 > of the new line, because it shared some characters with the old one?
 > Like when Emacs needs to replace "Foobar" with "Foobaz", and writes
 > only "baz" starting at the 4th character?

I can't remember but it's possible.  Unfortunately, I can't reproduce it
currently very well.  The attached screenshot is not representative of
what I mean.  It shows that the help echo "Insert ..." can start before
the position where the menu ends but there's already some other gruft
before it.  Believe me that I've seen an echo area where text like "rate
on its file" doesn't appear but the first help echo is written out
correctly, followed by some spaces, followed by the second help echo
written out correctly and the second help echo does not align with the
right border of the menu but starts on the left of it.

Reproducing things reliably is practically impossible.  Somehow, the
state after killing a terminal or terminating emacs in it seems to
affect the next invocation of the terminal and/or emacs.

martin