Further research revealed that this is a known issue with Slack in XFCE, since Slack does not properly expose its icon to the windows manager. The workaround is to set the icon manually using xseticon
. The links below detail the problem and the fix. In my case, it involved writing a script that ran after Slack opened and set the icon properly. A highly simplified version looks like this one from one of the mentioned links:
function change-panel-icon {
sleep $WAIT
activeWinLine=$(xprop -root | grep "_NET_ACTIVE_WINDOW(WINDOW)")
activeWinId="${activeWinLine:40}"
xseticon -id "$activeWinId" "$ICON"
}
$APP & change-panel-icon
The Spotify app has a similar issue, and it sounds like some other Electron apps do as well. Hopefully this will be fixed in a future XFWM release.
XFCE (Xubuntu) Alt-Tab switcher: missing app icons - Ask Ubuntu
XFCE (Xubuntu) Alt-Tab switcher: missing app icons
Missing icon and app/window not present in application switcher · Issue #37 · flathub/com.slack.Slack · GitHub
https://github.com/flathub/com.slack.Slack/issues/37
[SOLVED] Applications to the right and Slack icon problem / Desktop / Xfce Forums
https://forum.xfce.org/viewtopic.php?id=13149
Customize icon app on panel running apps and workspace ? / Desktop / Xfce Forums
https://forum.xfce.org/viewtopic.php?pid=45963#p45963
15510 – Icons for slack "desktop" application not visible in the window decoration and the task switcher
https://bugzilla.xfce.org/show_bug.cgi?id=15510
Slack snap - window has no icon - snap - snapcraft.io
https://forum.snapcraft.io/t/slack-snap-window-has-no-icon/3589/40