The tl;dr is that the current desktop client has been in maintenance mode for almost four years. It requires that an iOS phone be available, connected, unlocked, that Threema be in the foreground, and that the screen be on in order for the desktop client to function at all.
The successor—Threema Desktop 2.0—has been in development for about 4 years, has been actually available in beta for about a year, and is still so buggy and limited in functionality as to be barely usable. The bar set... [More]