small checkboxes (or other mark) next to items in selection lists, which don't go away when you click in the wrong place or accidentally try to add another one

small checkboxes (or other mark) next to items in selection lists, which don't go away when you click in the wrong place or accidentally try to add another one while not holding down ctrl. Seriously - when you have a large list of which you want to mark substantially some (say, 20%) of the items, some next to each other, and some not, the prevailing method of ctrl+shift+click suck horribly (and touch methods only slightly less). Back before mice were common, you would have a "mark/unmark this item and move to the next one" button (typically insert and/or space), a "unmark all" (typically insert), and it was simple and effective. It's still available in 7zip on Windows, but AFAIK that's basically the last bastion.