Michael -

> I will still need some time to find a final solution, but for the time
> being you could set tramp-use-ssh-controlmaster-options to nil before
> you open a remote connection. This shall prevent the problem. I would
> appreciate if you could confirm this.
[DG] Happy to.

I checked that I still see the issue (clicking on an error message suspends
compilation until ctrl-G) without this variable being reset.

I then restarted emacs and changed the variable, as confirmed by this
help-variable output:

tramp-use-ssh-controlmaster-options is a variable defined in ‘tramp-sh.el’.
Its value is nil
Original value was t


As you predicted, the issue does *not* show-up
once tramp-use-ssh-controlmaster-options has been reset to nil like this.

This was using the version of tramp that ships with emacs 27.1. The
compile.el patch we discussed earlier is applied. JFYI, the version of ssh
on my mac is:

$ ssh -V
OpenSSH_8.1p1, LibreSSL 2.7.3


Best regards,
Duncan.