Multiple select is imo even harder, because it’s not just that it’s not customizable, but also that multiple select is plain old bad UI in the first place. And while there are alternatives I personally don’t consider any of those universally good.
OpenUI however does truely seem to be able to make progress happen looking at the things they proposed that turned into standards.
I have only one such problem that’s scalable to the universe size! Just think that for years you have written macros that generates some functions/macros and documented the generated code inside quote do … end block and then you realize that you can simply use @callback attribute for documentation purposes (like for example Ecto.Repo is doing).
This was because the behaviours I remembered were pigeonholed for 3rd-party modules and not for the code you generate. I’m still in a process of rewriting all related projects.
That’s said the best topic is of course the never ending story: