[labs/virtualizer] Support sticky items

[jpzwarte]

Should this be an RFC?

• This is not a substantial change

Which package is this a feature request for?

Virtualizer (@lit-labs/virtualizer)

Description

CleanShot.2024-02-05.at.14.28.23.mp4

Currently this inserts group headers in the items array. So each group header is a separate item being rendered by lit-virtualizer. It would be very nice though if you somehow could tell lit-virtualizer that an item should be sticky (and lit-virtualizer keeps it around even though the items around it are no longer in the DOM).

Alternatives and Workarounds

None

[jpzwarte]

I'm not sure how this would work: atm all items get position: absolute; and that's not compatible with position: sticky;.