chat: an opened thinking row prints its first line twice #1636

Implementation: FlyBot (ours) — PR follows within 24h. Do not pick up without a pickup-ok comment on this issue.

Written-by: FlyBot Lead
Claimed-by: FlyBot Lead
Claim-Id: f1b0c1c8af10498cb45709de2bcf4589
Branch: dev/1636-chat-an-opened-thinking-row-prints-its-f
Base: 331dd62
At: 2026-10-29T05:01:30Z

Implementation: FlyBot (ours) — PR follows within 24h. Do not pick up without a pickup-ok comment.

A thinking row that is opened prints the same sentence twice: once as the grey preview inside the <summary>, once as the body text directly below it.

Written-by: FlyBot Lead

Reproduction (main 331dd620, the tree #1631 merged as)

core/src/client/cells.js (ThinkingRow, ~:133-146) puts the first line inside the summary:

h('details', { className: 'bt-think' },
  h('summary', null, h('span', { className: 'bt-think-caret', 'aria-hidden': true }, '›'), t('Thought it through'),
    peek ? h('span', { className: 'bt-think-peek' }, peek) : null),
  h('div', { className: 'bt-think-text' }, text))

details carries no open state and no onToggle, so .bt-think-peek is in the DOM in every state. core/src/client/styles.js (CONVERSATION_CSS, ~:890) only styles .bt-think[open] .bt-think-caret, and has no rule for the peek in the open state.

Reproduced against the real cells.js with an esbuild harness, reasoning text Looking at the store first\nThen the second line:

• details props: ["className","children"]
• summary text: ›Thought it throughLooking at the store first
• body (div.bt-think-text) starts with the same Looking at the store first

So an opened row shows a truncated grey copy of its own first line above the full text.

Expected

Closed: unchanged — label plus the one-line peek, which is what #1624 asked for.
Opened: the summary shows the label only (no duplicated sentence), the body shows the reasoning once.

Fix (scope)

One CSS rule in CONVERSATION_CSS, e.g. .bt-think[open] .bt-think-peek{display:none} — or, if you prefer, conditional rendering on toggle; either way the closed state must render byte-identically to today.

Out of scope: turn-reasoning.js, the empty guard, "live draws no row", the peek's wording/charset (that is a separate improvement: fence lines, grapheme-safe truncation, ellipsis).

Acceptance

• the open state's summary no longer contains the body's first line (one new test that sets/toggles the open state);
• the existing core/test/thinking-row.test.js cases (closed state, 5 cases from #1631) still pass unchanged;
• TZ=UTC ARCH_DOCS_BASE=<live main> npm run verify exits 0, ds-bot-patches check clean;
• regenerate generated artifacts only if the source change requires it and say so in the PR.