So to me it looks like some state/hint either on Xmonad's or Emacs' side
isn't refreshed or heeded when switching workspace.

I actually kind of like that Emacs frames on workspaces other than the
current one are considered invisible now, because of course they're not
visible :).

But switching back to that workspace should ideally make all frames
visible again.  I'm certain that there are both Xmonad and Emacs hooks
that allow the user to program this, but previously this wasn't needed.

BTW, I forgot to include in the recipe above that

  (mapc #'make-frame-visible (frame-list))

also changes nothing.  It seems that the only thing that deiconifies
frames is giving them input focus.

[ P.S. Let me know if fullscreen inline screenshots are annoying, and
  what to do instead.  I passed the images through pngquant so that at
  least their size is small. ]

> As soon as you have done that, I would like to give you a few
> instructions to make the code that broke this a NOOP and, in the worst
> case, make its execution optional on tiling WMs.

Thanks,

-- 
Basil

In GNU Emacs 28.0.50 (build 1, x86_64-pc-linux-gnu, X toolkit, cairo version 1.16.0, Xaw3d scroll bars)
 of 2021-05-13 built on tia
Repository revision: 43701a84367b76ccc93ad46f89110486988eec10
Repository branch: master
Windowing system distributor 'The X.Org Foundation', version 11.0.12011000
System Description: Debian GNU/Linux bullseye/sid

Configured using:
 'configure 'CC=ccache gcc' 'CFLAGS=-O2 -march=native' --config-cache
 --prefix=/home/blc/.local --enable-checking=structs
 --with-x-toolkit=lucid --with-file-notification=yes --with-x'

Configured features:
ACL CAIRO DBUS FREETYPE GIF GLIB GMP GNUTLS GPM GSETTINGS HARFBUZZ JPEG
JSON LCMS2 LIBOTF LIBSELINUX LIBSYSTEMD LIBXML2 M17N_FLT MODULES NOTIFY
INOTIFY PDUMPER PNG RSVG SECCOMP SOUND THREADS TIFF TOOLKIT_SCROLL_BARS
X11 XAW3D XDBE XIM XPM LUCID ZLIB