I was hoping we might be able to make the header row itself sticky, rather than the individual children inside it. However, it didn't work when I tried it, so this lgtm.

LOL, you actually tried what I wrestled with. I, too, found that weird.

Unfortunately, the select-all flow is a lot harder to support with position: sticky and CSS table (as opposed to <table>). I may just reorder the elements and will require different kind of changes. Will do that in a follow up.