[selectmenu] alternative UX to unselect/uncheck selected values

[NeilWix]

I think that the mechanism of a slot="button" with the selected values, poses some discoverability and operability challenges for keyboard users. I'd like to suggest an alternative: Instead of popping out "bullets" of selected options, sort the selected items at the top of the options menu and 'em as "selected items". A complementary mechanism should follow to inform users how many items are currently selected when the options menu is collapsed.