composer

One quiet, auto-growing editor shell shared by every dashboard-authored composer; each home supplies its own grammar and actions.

Command Box, TimelineChat, session launch prompts, issue replies, eval remarks, and a new issue's description are different product actions, but they should not each invent a textarea. They share one small editor shell: a quiet bordered surface, a borderless controlled textarea that grows from a useful floor to its home's cap, and a persistent action footer. The footer is part of the shell's geometry, so growth adds lines above it instead of moving the primary action around.

The shared primitive owns layout, textarea measurement, focus styling, disabled state, and the IME guard that distinguishes a composition commit from an ordinary Enter. It deliberately does not own domain meaning. Message homes decide that plain Enter sends and Shift+Enter adds a line; launch homes keep Enter as native long-form editing and submit only from their explicit action. Each home still supplies its placeholder, menus, triggers, attachment controls, send behavior, error copy, and draft lifetime. Issues and Evals keep the one ReplyComposer behavior in Thread.jsx; Command Box keeps its session control grammar in command-box; the Issues compose page (issues-view) mounts the same shell with its own action row and submit. Reuse stops at this natural boundary, instead of growing a parameter-heavy universal message form.

Textarea measurement follows the rendered box model. For a border-box editor the shared growth path adds its top and bottom borders to scrollHeight before clamping, so below the cap the visible client box contains the complete scroll box (scrollHeight <= clientHeight) rather than silently borrowing from bottom padding.

The surface uses the dashboard's existing type and color tokens, an at-most 8px radius, and no nested card. It has stable width and responsive constraints so menus, long words, progress, and errors cannot resize or spill it. A host may dock it in document flow or suspend it over a terminal; that placement is outside the primitive, while its internal editor/footer geometry stays identical.