Multi-select dropdown validation: a request the format cannot natively satisfy

Cluster: data-validation

Scenario

A user wants a cell where the reader can pick several values from a predefined list and have them accumulate in the cell, say comma-separated, rather than a normal list dropdown that replaces the cell content with a single chosen value. They ask the library to emit such a "multi-select dropdown".

Spec note, not a corpus case: there is nothing to assert implementation-blind, because a multi-select dropdown is not a native spreadsheet-format feature. Standard list data validation is single-select, and the accumulate-on-pick behavior in real spreadsheets is implemented with an event-handler macro (VBA Worksheet_Change) in a macro-enabled workbook, not with any writable validation XML. Recording the constraint and the portable primitive we should nail is the durable value.

Desired behavior

• Set the expectation honestly. The library cannot emit a self-contained, macro-free multi-select dropdown that works on open in Excel, because the format has no such construct. Public docs and types should make that clear rather than implying a multiSelect: true flag could exist.
• Nail the portable primitive instead. First-class, well-typed single-select list validation is the thing every consumer of this request actually shares underneath. Inline literal lists, cross-sheet range sources, and defined-name sources must all round-trip cleanly, already covered by corpus cases. A robust single-select API is the honest 90% of the ask.
• If a multi-select affordance is ever offered, it must be an explicitly opt-in, clearly-labelled macro-emitting path (a macro-enabled .xlsm with an injected change handler), never silent, never in a plain .xlsx, and never presented as a native validation type.

Open questions

• Is emitting a macro-enabled workbook with an injected change handler ever in scope, or permanently out? It collides with the no-unsafe-eval and minimal-surface stances and with macro-security concerns.
• Would a helper that pre-populates a helper column plus a single-select list cover the common intent, pick-from-list with one value per row, without any macro?
• Documentation placement: a "format limitations" section so this recurring request has a canonical answer.

Related: list-validation-inline-formula-length-limit, no-unsafe-eval-csp-compatible, sheet-protection-password-hash-compatibility (macro and security posture), cross-sheet-list-validation-x14.