Comment 16 for bug 1827302

As original reporter, I can confirm that the xprop trick works around this problem.

I used a modification of the wrapper script from #12, after

sudo apt install wmctrl
# as of recent https://github.com/xeyownt/xseticon/pull/4
sudo snap install xseticon

and adjusting the icon path to use the snap

ICON=/snap/slack/current/usr/lib/slack/resources/app.asar.unpacked/src/static/slack.png

and using this for the xseticon binary path:

/snap/xseticon/current/bin/xseticon