fwiw, Windows does have an API (called uxtheme, I believe) that you can use to draw native widgets.
I guess uxtheme is unsuitable for win32k lockdown for some reason, probably because it's GDI-based.
[1]: https://devblogs.microsoft.com/oldnewthing/20050211-00/?p=36...
You’re right, uxtheme can’t be called directly from content because of win32k dependencies. One option could be for content to request the parent to draw those controls, but as you can imagine there are drawbacks to doing that. As usual, there are a lot of trade offs involved.