I also hate that the 'official' desktop 'app' is so slow and have started implementing a Qt-based mattermost client:

https://github.com/nuclear868/mattermost-qt

At the moment it supports basic features like sending and receiving messages and viewing images.
With the time it will get better.

+1 for the Telepathy Bridge.
https://wiki.freedesktop.org/telepathy/

In matterfront the notifications and the Tray icons are working just fine, you just need to enable them: https://github.com/HackerHappyHour/matterfront#enabling-the-tray-icon

Totally agree with Dev Null. Quality of notifications from the client is currently the top negative comment i hear in my company. Anything that can be done in this space would be a big plus

All of these Windows clients currently have the same problem regarding notifications. There is no persistent task bar flashing or any other persistent mechanism to notify the user if they were away from their computer when a message came in. The one-time 'ding' sound is not sufficient if you're away from your desk.

How do you interact with a mattermost server? I think having a native gui app would be great, and will provide a better integration with for instance GNOME online account services.

A possible way to have a Linux client would be to implement the protocol for Telepathy.
Any client based on telepathy (for example empathy) would be able to communicate in mattermost channels.
I do realize some features could be missing, but it is probably worth a try.

Just an update, there's a new community project, Matterfront, that's a desktop app for Mattermost: https://github.com/lloeki/matterfront

Seems promising!

https://developer.mozilla.org/en-US/Marketplace/Options/Open_web_apps_for_desktop One possible way to do this for folks.

Just use QT and and have the same code base for all platforms

There's an open-source project that runs Slack on a Linux desktop: https://github.com/raelgc/scudcloud/

Electron would be great for this.

What do you think of writing a cross-plattform app in http://electron.atom.io/?

It is the same open source stack which is used by Slack, Atom (Editor) and even Microsoft: https://code.visualstudio.com/

+1

Why would anyone want to waste RAM and CPU cycles on a browser just for a messaging app? And, native notifications! :D