stray stdout/stderr/pseudo console garbage output

[Thomas Wolff <towo@towo.net>]

When I run mintty in the background from a nested shell, like
     mintty & within a shell script, or
     (mintty &) from an interactive shell,
and it produces the following error output, some stray string is 
appended to its output, like:

Ignoring unknown option 'ShowTabBar'.
Font not found, using system substitute:
Luxi Monotch??????                ffer: pty0: pcon

or:

Ignoring unknown option 'ShowTabBar'.
Font not found, using system substitute:
Luxi Mono      2 [main] mintty 1551 
fhandler_pty_slave::push_to_pcon_screenbuffer: pty0:
pcon_attach_to mismatch??????

The first message ("Ignoring ...") is written with fprintf(stderr, ...).
The second message ("Font not found") is written with fputs(..., stderr).
Without specific mintty configuration, reproducible with the command 
(parentheses needed):
(mintty --fn "dummyfont" -o dummyopt=none&)

Observable with 3.1.0, 3.1.2 or snapshot 2020-01-31.
Sorry, I could not reduce the test case to a simpler setup.
Thomas

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