There's also the Emacs Widget Library[1] (which I hoped this posted would be about). It's a plain text widget library that's quite powerful. Unfortunately (and surprisingly for Emacs), the documentation for it could use some love.

My understanding of the Widget Library is that it attaches various keywords and plists to a symbol which is considered the "widget". The library otherwise consists of functions that expect certain keywords on the "widget" symbol in order to perform actions or to be drawn on the screen.

The challenge is, the documentation doesn't clearly lay out what the keyword API is. This makes it hard to compose widgets in ways beyond what's shown in the docs.

[1] https://www.gnu.org/software/emacs/manual/html_mono/widget.h...