TL;DR
All "small" calculator softwares I know share the same design flaws. And the GUI is not the only stumbling block in their poor efficiency. Handling correctly numerical approximations - expected or not - is crucial to trust the results. It is usually not done in lightweight tools. It should. So I prove that if I could do it, OS manufacturers could as well.
However, I wrote a full article explaining why I had to build something "new" that does not behave like usual desktop calculator softwares :
<https://chachatelier.fr/chalk/article/chalk.html>
TL;DR All "small" calculator softwares I know share the same design flaws. And the GUI is not the only stumbling block in their poor efficiency. Handling correctly numerical approximations - expected or not - is crucial to trust the results. It is usually not done in lightweight tools. It should. So I prove that if I could do it, OS manufacturers could as well.