fix(web): 回答从开头开始读，两个会话面同一套钉顶跟随

frontend/DESIGN.md

@@ -223,11 +223,21 @@ Radii have two visual steps. Controls use 8px (`--radius-md`; `--radius-xs` and
 - **Accessibility:** persistent label where practical; composer has an explicit accessible label.
 - **One composer:** the rectification surface renders the same `ChatComposer` as the main chat; there is no second composer. A surface that owns its own draft passes it as `value`; the main chat reads the draft store. Both count down from the same 500-character ceiling through `CharacterRemaining`, which the composer renders inline and only once `characterRemainingVisible()` is true — it no longer has a permanent container. Controls that belong to the composer go through the `toolbar` slot; nothing is stacked below it.
 
+### Answer start anchor
+
+Both chat surfaces pin a new turn at its head instead of following the last streamed token.
+
+- **Head:** the user row of this turn. When this turn has no user row (rectification choice tap, opening, auto-continue) the head is the new assistant row.
+- **Pin:** on send (and on those rectification new-turn entries) the scroller places that row at the top with a `space-4` gap and then holds still while the reply grows downward.
+- **Spacer:** the last assistant row's `min-height` is `calc(var(--conversation-viewport) - var(--latest-turn-head-height))`, both variables written onto the scroller by `useConversationScrollAnchor`. Not sticky, and `.conversation`'s `padding-bottom` is unchanged. The spacer lasts until the next send; switching sessions clears it.
+- **History:** opening another session still lands on the newest content once.
+- **Jump to latest:** if the growing turn (or the reader scrolling) leaves more than 96px below the fold, the shared chip appears; pressing it sticks to the bottom for the rest of the turn.
+
 ### Jump to latest
 
 - **Structure:** one pill button (`JumpToLatestButton`) with a down arrow and the label “跳到最新”, shared by both chat surfaces.
 - **Placement:** hangs off the composer wrap's top edge (`.jump-to-latest`, absolute, `bottom: 100%`) so it never reflows the transcript or the composer. It occupies a 56px band above the composer — the 44px target plus the `space-3` the overlay holds under itself — and the transcript has to reserve that band **plus** one more `space-3` below its last item, or the last tappable row reads as covered and its centre is where the chip's only `pointer-events: auto` area sits (BUG-919). On the rectification surface that reservation is `.message-list { padding-bottom: calc(var(--rectification-jump-clearance) + var(--space-3)) }`, and `.conversation` carries the matching `scroll-padding-block-end`. Full non-overlap at every scroll position would cost `conversationAnchorThreshold` (96px) on top of the band, i.e. 152px of trailing void; that was rejected — the chip only appears once the reader has scrolled up past that threshold.
-- **Visibility:** shown when the reader is not anchored — more than 96px from the bottom after scrolling up — and hidden again once they return or press it. Sending a question re-anchors.
+- **Visibility:** shown when the last turn extends more than 96px below the fold — either because the reader scrolled up, or because this turn grew past the viewport while pinned at its head — and hidden again once they return to the bottom or press it. Sending a question pins the turn head; pressing the control re-anchors to the bottom.
 - **Surface:** canvas fill, hairline border, `--shadow-elevated`; hover uses the warm card surface. No utility-class shadows.
 - **Accessibility:** a real button in document order with a visible label matching its accessible name, 44px target, and the focus ring; the icon is decorative.