Short version, so you do not have to click:

flex: 1 expands to flex: 1 1 0%. That 0% is a percentage basis and resolves against the parent's height. A panel that is height: auto under a max-height cap has no definite height, so the percentage has nothing to resolve against and the child contributes zero. flex: 1 1 auto fixes it, because the basis becomes the content's own height.

The reason it took me an evening: Chromium quietly does what you meant, and the engine that broke it was WKWebView. Every test was green because Playwright ships its own WebKit build rather than the system one.

I have not worked out whether the spec requires that collapse or merely permits it, so I cannot say which engine is wrong. If you know which reading applies to a max-height-capped auto-height parent, I would like to hear it.

With the new release of 3.10.2 I wanted to look a bit more closely into Pandoc and all those different markup languages out there - and what converts with how much "loss".

Maybe you also had to recently convert Markdown to HTML, or the other way around. In my case even different textual files into PDFs.

One key finding:
Modern languages that work with an AST are definitely competitive here.

Maybe you find it also interesting. I actually didn't know most of those languages.

Fun fact:
The author of Pandoc is also one of the original Markdown standardization authors 15 years ago (well, if you can call it that, since it evolved still into quite the chaos) - as well as Djot, which is now only a few years old and supposed to be a markdown successor of sorts.

I am mainly interested in shaping "the" markup language of the future, that is a good compromise of readability and writeability for humans, but also consumable for machines, contains all the important elements to express relevant documents from offline to online.
As a programmer I am also a big fan (and in need) of dogfooding, of course :)