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Research: Fix Thinking Panel Re-Expand on Conversation Switch

Feature: 095-fix-thinking-panel-expand
Date: 2026-03-18

Phase 0 Summary​

No open "NEEDS CLARIFICATION" items. The spec and codebase are sufficient to implement. This document records the chosen approach and rationale.

Decision 1: Where to derive initial state​

Decision: Derive the initial value of the thinking-panel expanded state inside the component that owns it (ChatMessage), using a lazy useState(initializer) so the value is computed once per mount from current props.

Rationale: The bug is that on remount (e.g. after switching conversations), the component re-runs and previously used a constant initial value. Making the initializer depend on message.isFinal and the user default ensures that (1) final messages default to collapsed, and (2) streaming messages still use the user preference, without introducing global or cross-component state.

Alternatives considered:

  • Persist expand/collapse per message ID in URL or store: Rejected for scope; spec explicitly leaves per-message persistence out. Can be a follow-up.
  • Move state up to a parent and pass down: Rejected; state is local to each message and remount is the norm when switching conversations; the fix is to make the initial state correct on each mount.

Decision 2: Lazy initializer vs useEffect​

Decision: Use a lazy useState(() => ...) initializer, not a useEffect that sets state after mount.

Rationale: The correct initial paint is "collapsed" for final messages. A lazy initializer gives that on first render and avoids a flash of "expanded" then "collapsed". React docs recommend lazy initializers when the initial state depends on props.

Alternatives considered:

  • useEffect to set collapsed when message becomes final: Would still show expanded on first paint after navigation; adds an extra render and possible flicker.

Decision 3: Test strategy​

Decision: Rely on existing ChatPanel Jest tests; add or extend a test that asserts for a final message the thinking section is collapsed by default (e.g. collapsed or not expanded when message.isFinal === true).

Rationale: Constitution requires test-first; the existing "Thinking section (showRawStream)" tests cover default expanded. Adding a scenario for final messages ensures the new behavior is locked in and prevents regression.

Alternatives considered:

  • E2E only: Less precise and slower; unit test is sufficient for this state logic.
  • No new test: Rejected; constitution gate VII requires acceptance criteria to become test scenarios.