Thank you @Sandmonster,
The community member helping troubleshoot doesn’t have Linux to test in but this is his response:
I’m afraid I have no idea why this doesn’t work.
But I’d like to guess some more, if you don’t mind.
I don’t have a Linux desktop to test this, but: maybe--hiddenminimizes to the taskbar (I think? I’m talking about that thing with a rectangle for each opened program) and not to the traybar (that other thing in the lower right corner; you can configure Mattermost desktop to show a icon there). This “i3” thing has no such taskbar-thing, apparently (see screenshot; no taskbar, but a tray bar). So, you can’t minimize Mattermost to it. But again, just another guess.
Hope this helps!?