I’m embodying an archetype here, but I can’t resist.

The use of QTermWidget makes this almost entirely an exercise in building a hello-world Qt app and very little about building a terminal. That is all very well if you are looking to learn Qt—which is by no means an unworthy aim—but I wouldn’t use the adjective “minimal” to describe any Qt app, no matter how short its main().

If you are actually looking for a minimal terminal, whose source code is intelligible and readily customizable, look no further than https://st.suckless.org/

Good suggestion! I found st a joy to read. Only a couple of C files, few frills and a straightforward coding style. I can see what the suckless folks are getting at.

Slightly OT here but I was thinking about what a suckless UI system would be like - one without excessive layering and as few moving parts as possible. I'm fairly sure it should be possible by avoiding truetype fonts and using the framebuffer directly. That would preclude ubiquitous animation but smooth scrolling and dragging is perfectly doable even with a single buffer.

What about IMGUI? https://github.com/ocornut/imgui