Draw.io is quite good, and it is made without using any frameworks. Alas, the source code is not open to learn from.
<https://www.quora.com/What-is-the-tools-and-languages-used-i...>
<https://www.quora.com/What-is-the-tools-and-languages-used-i...>
What about this: https://github.com/jgraph/drawio
That repository only contains the minified code, not the original source.
Like a "proof that open source is working", a show case of a good quality, open source project.
Oh well, time to strike that one off the list.