The reason my WM did not have stderr/stdout available is, I had started it from a terminal, disowned it, and then closed the terminal. So, stderr was pointing at /dev/pts/2, which had been deallocated.
That's probably not an entirely original way to shoot oneself in the foot; I'm sure I've done that plenty of times before and just didn't try to run anything from the WM that expected to be able to write to stderr. -- see shy jo
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