Update!
This post has obsolete information. Read: Well, that was fast: Slack 1.1.8 reverts system tray functionality.
Original Post…
TL;DR
Version 1.1.5 of Slack’s Windows app no longer allows minimizing to the system tray. As long as Slack is running, its taskbar icon is visible. I didn’t like that very much, so I wrote an application - dubbed Headless Slacker - that enables users to choose at their own discretion whether or not to show Slack’s taskbar icon. It’s free and open source, so enjoy! Click the link above for installation instructions along with other details about the application.
I love Slack. I think it’s an outstanding piece of software, especially compared to its nearest competitor HipChat. When my suggestion to dump HipChat for Slack at work gained traction I was both relieved and excited. But (you knew a ‘but’ was coming, didn’t ya!) right from the get-go I had one relatively minor quibble with their Windows application. Namely, the taskbar flashed orange when you received new message notifications. That I found distracting and visually jarring. I sent Slack some feedback on it, hoping the taskbar flashing could be made optional. Thus far, that particular idea hasn’t been implemented. My guess is that most users aren’t that bothered by it or - on the opposite end of the spectrum - actually like it. Either way, it wasn’t that big of a deal because if you checked the Leave app running when the window is closed option, you could minimize the application to the system tray. No taskbar icon == no orange flashing.
Then version 1.1.5 came out.
From the sounds of it, Slack was getting complaints from users who were chronically missing important messages. How, I honestly don’t know. Between desktop notifications, the subtle - but very real - change to the system tray icon when you have unread messages, and mobile notifications, I never found myself missing messages. But that’s neither here nor there. For other people it was a problem. Got it. So in version 1.1.5, the Slack Windows application was changed such that it is permanently affixed to the taskbar, like it or not, rain or shine. All the Leave app running when the window is closed option did now was turn the window’s close button into an additional minimize button. From this point forward, there was no escaping the jarring orange flashing associated with new messages. Aaarrggghhh!
After that change I really really wished I had an option to disable the orange flashing. I also didn’t like having to forfeit valuable taskbar real estate for Yet Another Chat Application. Then I started thinking back to the days of yore when I did lots and lots of Windows API programming and it occurred to me: there must be a way to forcibly remove an application icon from the taskbar! Indeed, there is. My next thought was: Muahahahahahah! A few hours later, I had a working version of Headless Slacker.
Checkout the readme for more details, but basically Headless Slacker adds a jumplist menu item to Slack that allows you to hide the taskbar icon. To unhide the icon, just single click the Headless Slacker icon (a red ‘@') floating around in your system tray. Headless Slacker runs quietly in the background when Slack is hidden from the taskbar and exits when Slack is shown on the taskbar. If Slack is not running when you click the Headless Slacker icon, Slack will be started up.
Enjoy!