The new mattermost desktop app use a custom window border.
But this does not always meet user expectations, especially under KDE I can freely configure the border.

In my case, I have the close button on the left side (like MacOS) and no minimize / maximize buttons.

I know from some apps (e.g. Chrome) that there is an option to use the framework of the OS.

https://www.imagebanana.com/s/1676/KOA32QBI.html