# AAC sub-spec — Motor stability

Normative refinement of [SPEC §3](../SPEC.md). Reference: `aac_core::board::Cell::{position, locked}`.

A configured board's most valuable property is not its content but its **geometry**: the learned muscle memory of where each word lives. A user who reaches for "want" in the top-left does so without looking, often without conscious thought. Anything that moves "want" destroys that — and motor relearning for a person with a significant physical impairment can cost weeks. This sub-spec makes geometry an invariant, not a preference.

## 1. Positions are immutable

A cell's `position` MUST NOT change once assigned, for the life of the cell, except by an explicit user or editor edit to a cell whose `locked` flag is `false`. In particular, the following MUST NOT move a cell:

- prediction or frequency (`Cell::frequency` weights ranking, never placement);
- context, time of day, partner, or activity (any L3+ enrichment, [`cascade.md`](cascade.md));
- language switching (a code-switch is a traversal, not a re-layout — [`access_model.md`](access_model.md));
- "tidying", auto-fill, or alphabetization.

## 2. Locked boards do not reflow

A client MUST NOT reflow, resort, auto-arrange, or re-pack the cells of a board whose cells are `locked`. Adding a cell to a locked board MUST place it in a free position without disturbing any existing cell.

## 3. Hiding is in place

Progressive disclosure — hiding advanced vocabulary while a user is still learning — MUST be done by rendering a cell visually inactive **at its position**, never by removing it and collapsing the grid. A word that reappears later MUST reappear in the exact position it would have occupied all along. This is why `CellContent::Blank` exists: it reserves geometry against a future word, and why a cell can be marked `hidden` (proposed, [SPEC §2.1, §3.1](../SPEC.md)): the word stays where it is, not selectable, until it is shown again.

Editing follows the same rule ([SPEC §3.1](../SPEC.md)): an edit changes one cell in its place; a new cell goes only into a free position, a removed one leaves its position free, and a grid grows or shrinks only at its end, where it is empty.

## 4. Determinism across clients

Two openings of the same board — on the same client, on a different conforming client, before and after an interchange round-trip (§7–§8) — MUST present identical cell positions. Position assignment during ingest of a positionless source MUST be deterministic (row-major fill) and then fixed (see [`interchange.md`](interchange.md)).

## 5. Conformance

An implementation that violates §1–§4 is non-conformant regardless of any other quality. Motor stability is checked by opening a vector board, applying a frequency/prediction update, re-serializing, and asserting position-set equality against the input.