Modify button part behavior for <select multiple>?

[dandclark]

In the <select> anatomy, there are a couple of spots where we don't modify open or aria-expanded if the multiple attribute is set, e.g. see listbox keydown(enter). However for button there's nothing to indicate that it should not toggle these attributes, or that it should not appear at all, if multiple is present. So there's an inconsistency here.

Adding an extra layer of difficulty is the fact that for Safari/Chrome/Edge on iOS there is actually still a visible button for <select multiple> that opens a popover with checkable options. So, the button not affecting the open state of the popup if multiple is set isn't even universally true if we consider iOS.