Consider adding auto-sizing for <select> as a listbox

[zcorpan]

Similar to #6807 but for <select multiple> / <select size=2>

There's no way without JS to specify that a listbox select should have the same height as its contents.

Anecdotal example of a requests for this (found by searching "auto-size select multiple"):

https://stackoverflow.com/questions/9943886/make-multiple-select-to-adjust-its-height-to-fit-options-without-scroll-bar

Do UI libraries typically have a "<select> as a listbox" widget? For those that do, can it auto-size?

cc @whatwg/forms @mfreed7

[annevk]

This also sounds like the kind of thing I'd like to leave to CSS. cc @pxlcoder @whatwg/css