Mattermost's clients are awful. Just awful. The phone apps are effectively unusable (incredibly buggy and suck down battery like no tomorrow), and the desktop electron app is a complete hog. It doesn't even handle its faux tabs properly (takes half a second to switch, and it resets the position to the end every time you change a tab, which is... dumb), and the scrollback handles high-volume channels very poorly (I have to click "Load more messages" at least 20x times every time I want to scroll back to the beginning of the day). Now imagine every time you switch tabs you have to click "Load more messages" a bunch of times to see earlier messages because it resets when you switched tabs.
If this is the future of messaging, then holy shit. It's almost a parody.