ConstThe list holds values and nothing else. Over them, pinned above the scroll, is the root: one
tri-state checkbox for every row in view, labeled by the count of checked values over the
values in view. Add, for a host that takes typed values, sits after the search field. A blank
is a value: a host that has one lists it last as its blank row, which the root and the count
cover like any other.
Reads no cells: the host supplies the items and owns the checked set (by text). The search narrows the VIEW only: the root acts on the rows in view, rows narrowed away keep their state, and what the host applies is the whole set (Sheets' rule, not Excel's replace-with-results). The list has the focus on open: Up/Down move its highlight, Space toggles the highlighted rows, Enter applies, Tab leaves it, and a typed character moves to the search field. From the field the same keys reach the list, Enter applies, and Escape clears the search and then closes.
With dates, the leading items are listed as a tree (Excel's year › month › day): a node's
checkbox is the state of its items, its row toggles them all, and its disclosure (or Left and
Right on the highlighted row, with nothing typed) folds it. Collapsing hides the member rows
from the view; they keep their state.
The root wears the grid's surface and mode, so the list looks like the cells it stands for even inside chrome of another color.
A searchable checklist of values under a tri-state Select All — the filter dropdown's Values body, and any other "which of these" choice.