bug#21718: 24.5; Backgrounding emacsclient hangs emacs

[Ed Avis]

I've gone back to the original recipe, with no GNU screen and no .bashrc file 
to set environment variables.  It's as I described it back in 2015:

I am running emacs from a text ssh session.

% emacs -Q
M-x server-start RET
M-x shell RET
% touch foo
% emacsclient foo
C-x b RET                  (switch back to shell buffer)
M-x comint-stop-subjob RET

I expected this to background the emacsclient process within the shell
window.  But it causes all of emacs to disappear from the screen, back
to the original shell prompt that launched emacs, where the emacs
process appears to have hung and can only be killed with C-c.

This is emacs-27.1 running on RHEL 8.4.  The build was originally derived from 
Fedora Linux but I rebuilt it on RHEL.  Do you want me to try a completely 
vanilla build from the tarball?

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