How to fix the Slack icon in Debian - XFCE?

22 April 2021linux

For a while I've been really annoyed that some applications (Slack, React-Native Debugger and some others installed via .deb) appear without an icon in the XFCE task list panel.

It seems to be a bug in the app itself or maybe Electron apps on Debian, and it even happens when installed with SnapStore. Either way, it annoys me.

I spent a lot of time researching and learned that the Window Buttons item gets its configuration from programs’ .desktop files. Searching for Slack's file gives:

$ sudo find / -iname "*slack*.desktop"

# Output
/home/veuge/.config/autostart/slack.desktop
/usr/share/applications/slack.desktop

The file in $HOME/.config/autostart is a symlink to /usr/share/applications, so fixing that one is enough. Its content is:

$ cat /usr/share/applications/slack.desktop

# Output
[Desktop Entry]
Name=Slack
StartupWMClass=Slack
Comment=Slack Desktop
GenericName=Slack Client for Linux
Exec=/usr/bin/slack %U
Icon=/usr/share/pixmaps/slack.png
Type=Application
StartupNotify=true
Categories=GNOME;GTK;Network;InstantMessaging;
MimeType=x-scheme-handler/slack;

The interesting line is Icon=/usr/share/pixmaps/slack.png; the image exists and has correct permissions.

$ ls -l /usr/share/pixmaps/slack.png

# Output
-rw-rw-r-- 1 root root 38884 Mar 15 20:12 /usr/share/pixmaps/slack.png

Since config looks good, it seems like an ElectronJS bug, so there must be another fix. I found this Reddit discussion; the second comment says the workaround is to reset the icon after Slack opens, with two steps:

Get Slack window ID using xwininfo.
Set the window icon using xseticon.

Manual process:

$ xwininfo -root -tree | sed -e 's/^ *//'| grep -E '0x.*Slack\s{1}\|' | awk '{print $1}'

# Output
0x4400005

$ xseticon -id 0x4400005 /usr/share/pixmaps/slack.png

After running xtseticon the result is:

As Reddit says, doing this manually every Slack launch is suboptimal, so we tweak Exec in /usr/share/applications/slack.desktop to update icon automatically once Slack is known started.

I replicated the approach from this comment in a script, and then call that script from slack.desktop.

Script breakdown:

# Permite sobreescribir el .desktop
env BAMF_DESKTOP_FILE_HINT=$SLACK_DESKTOP

# Comando que abre Slack minimizado y sin output
/usr/bin/slack -u %U & > /dev/null

# Pausa para esperar que Slack inicie
sleep 10

# Obtiene window ID de Slack y lo guarda en la variable app_wid
app_wid=$(xwininfo -root -tree |sed -e 's/^ *//'|grep -E '0x.*Slack\s{1}\|' | awk '{print $1}')

# Actualiza el ícono dado el window ID y el icono en png.
xseticon -id $app_wid $SLACK_ICON

And finally, the modified slack.desktop:

[Desktop Entry]
Name=Slack
StartupWMClass=Slack
Comment=Slack Desktop
GenericName=Slack Client for Linux
Exec=sh -c /home/veuge/slack-init.sh
Icon=/usr/share/pixmaps/slack.png
Type=Application
StartupNotify=true
Categories=GNOME;GTK;Network;InstantMessaging;
MimeType=x-scheme-handler/slack;

Conclusion

This is a workaround to restore Slack/other app icons in XFCE; the real cause should be diagnosed.
It works when Slack is started manually or via auto-start.
The script can be improved to make it configurable and reusable for other applications with this icon bug. (I'll do that someday).