refactor(chat): publish composer geometry, not a bare measurement

[chrisnojima]

Problem

input-area/normal/max-input-area-context.tsx was three lines — its entire interface was React.createContext(0).

It published a raw measurement where callers needed a geometry model, so five modules re-derived that model from the integer plus their own inset and keyboard hooks:

• {closed: -insets.bottom, opened: 0} appeared byte-identical in normal/index.tsx:147 and suggestors/index.tsx:457, the second with a 4-line comment explaining it must mirror the first.
• maxInputArea * 0.35 appeared independently in input.tsx:1263 and command-markdown.tsx:20.
• - inputAreaHeight - 15 appeared twice ~35 lines apart in input.tsx, with 15 a bare literal both times.

Two shipped fixes three weeks apart edited the same file to re-sync these derivations. Zero tests covered any of the arithmetic.

Change

One pure computeComposerBox() owns every derivation and every constant, now named: composerPanelHeightRatio, expandedInputTopGap, commandMarkdownFallbackMaxHeight, singleLineHeight, threeLineHeight.

The context is split along a stable/layout seam, so consumers only re-render for what they read:

• ComposerAnchorContext — {bottomInset, stickyOffset, keyboardHeight, keyboardProgress}. Nothing layout-derived, so its identity survives every measurement. ListArea reads this and only this.
• ComposerBoxContext — the measured box and sizes derived from it. Read only by the composer's own panels.

The invariant is stated on the type: "Do not add layout-derived fields." Keyboard values stay reanimated shared values — nothing became React state.

maxInputArea and 0.35 now have zero occurrences outside the geometry module.

Two disagreeing derivations, both preserved

commandMarkdownMaxHeight is unclamped with a 250pt backstop; the suggestion list is clamped to [120, 240] and reserve-capped with a 0 fallback. At h=200 these give 70 vs 16. Preserved as-is rather than silently unified.

Dormant code, deliberately left dormant

MobileSuggestionArea's height clamp has never executed — @gorhom/portal renders it at a host outside the router, so the provider never reaches it and the height has always been undefined. Preserved exactly; switching it on is a real behaviour change. emptyViewport reproduces master's out-of-provider fallbacks exactly (250 / 0 / 36 / 78).

Validation

lint:all clean — 0 bailed out, 0 whole-props deps, tsc clean both projects.
jest --runInBand — 232 suites / 2257 tests (baseline 230 / 2236).

New composer-geometry.test.ts (18 tests) and composer-viewport-context.test.tsx with render-counting probes proving measuring the box does not re-render the list.

Note on mutation testing here: babel-plugin-react-compiler runs in the jest config and re-derives memoization from what an object literal actually reads, discarding a deliberately-broken useMemo dep. A mutation aimed at a dep array can false-pass; it has to change what the code reads.

[Copilot]

🟢 Approval recommended

The refactor is cohesive, removes duplicated arithmetic safely, preserves documented behavioral differences, and adds targeted tests to lock in the geometry and render invariants.

Pull request overview

Refactors the mobile chat composer sizing logic to publish a structured “composer geometry” model (instead of a single raw measurement), centralizing all derivations/constants and splitting the published values across two contexts to avoid unnecessary message-list re-renders on layout measurement/rotation.

Changes:

• Introduces composer-geometry as the single source of truth for composer box sizing, panel heights, and keyboard/inset-derived helpers.
• Replaces the old MaxInputAreaContext with ComposerAnchorContext (stable, non-layout-derived) and ComposerBoxContext (layout-derived sizes).
• Adds focused unit/component tests covering geometry arithmetic and the “anchor doesn’t re-render on measurement” invariant.

File summaries

File	Description
shared/chat/conversation/normal/index.tsx	Provides the new anchor/box contexts and uses computeComposerBox for measured layout-derived sizing.
shared/chat/conversation/list-area/index.tsx	Switches list keyboard/inset math to read from ComposerAnchorContext and uses geometry helpers.
shared/chat/conversation/input-area/suggestors/index.tsx	Uses ComposerBoxContext/geometry helpers for suggestion area sizing while preserving portal fallback behavior.
shared/chat/conversation/input-area/normal/max-input-area-context.tsx	Removes the now-obsolete raw measurement context.
shared/chat/conversation/input-area/normal/input.tsx	Uses ComposerBoxContext + expandedInputMaxHeight for expanded input and suggestion list sizing.
shared/chat/conversation/composer-viewport-context.tsx	Adds the two new contexts and defines their non-conversation fallbacks.
shared/chat/conversation/composer-viewport-context.test.tsx	Adds render-probe tests validating context split and render behavior.
shared/chat/conversation/composer-geometry.ts	Adds centralized geometry model/constants and worklet-safe helpers.
shared/chat/conversation/composer-geometry.test.ts	Adds unit tests locking in the shipped arithmetic and edge cases.
shared/chat/conversation/command-markdown.tsx	Switches command-markdown max-height to the new geometry-derived value.

Review details

• Files reviewed: 10/10 changed files
• Comments generated: 0
• Review effort level: Lite

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