Signum! was highly opinionated. It ran on the Atari ST but did its own thing for the user interface. You could access a lower layer of drawing primitives and obviate GEM. In those days multitasking did not exist.

There were a good number of these kinds of application back then. Steve was one, GFA Basic another.

Ah, it was actually STeve:

https://ataricrypt.blogspot.com/2024/03/steve.html

An application that was more a random selection of tools than a cohesive whole but some people swore by it.