fix(web): drop the branch action from user and steering bubbles

The branch control on user and consumed-steering bubbles could enable
only when a completed turn ended with no node after the message at all,
so readers saw a permanently unavailable control whose tooltip promised
a state it could not reach. An enabled one would mislead anyway: a fork
at a message seq cuts at the containing turn/end and takes the answer
along, the opposite of the branch-to-re-ask reading a control on one's
own bubble suggests.

MessageItem loses its fork props, PendingSteeringBubble loses the
showBranch special case, and messageBranchSeqs narrows to
assistantBranchSeqs: only a completed turn's transcript tail that is
the turn's own content-text assistant may fork. A steered turn keeps
its fork point under the settled answer, because fork is a log-prefix
cut and the steer is model-visible history the child inherits.

Web aria goldens drop the user-bubble disabled-branch row and its
hidden explanation text; the nested-subagent golden also loses the one
enabled user-tail fork handle, a loss the decision note accepts.

packages/client/ui-conversation/src/client/chat/ChatView.tsx

@@ -30,7 +30,7 @@ import type {
 import type { SnapshotSelectorHook } from '@deepseek-ai/dsh-client-ui-slots'
 import { IconChevronDownOutline14 } from '@deepseek-ai/dsh-client-ui-primitives'
 import type { ChatViewSlotProps } from '../contract/slots.ts'
-import { assistantActionsSeqs, deriveChatFlow, messageBranchSeqs, runningTurnStartTime, type ChatFlowItem } from './chat-flow.ts'
+import { assistantActionsSeqs, assistantBranchSeqs, deriveChatFlow, runningTurnStartTime, type ChatFlowItem } from './chat-flow.ts'
 import { AssistantMarkdown } from './AssistantMarkdown.tsx'
 import { GenericCommandCard } from './GenericCommandCard.tsx'
 import { GenericToolCard } from './GenericToolCard.tsx'
@@ -362,7 +362,7 @@ export function ChatView({
   // mid-turn text and every node of a running turn omit `time`, so
   // AssistantMarkdown stays chrome-free until the answer settles.
   const actionSeqs = useMemo(() => assistantActionsSeqs(nodes, turnEnds), [nodes, turnEnds])
-  const branchSeqs = useMemo(() => messageBranchSeqs(nodes, turnEnds), [nodes, turnEnds])
+  const branchSeqs = useMemo(() => assistantBranchSeqs(nodes, turnEnds), [nodes, turnEnds])
   const runningTurnStart = useMemo(() => runningTurnStartTime(turnTimings), [turnTimings])
   const turnMetrics = useMemo(() => deriveTurnMetrics(nodes), [nodes])
 
@@ -632,8 +632,6 @@ export function ChatView({
       <MessageItem
         node={node}
         retryActive={node.kind === 'model-retry' && node.seq === activeRetry}
-        onFork={forkAt}
-        forkUnavailable={!branchSeqs.has(node.seq)}
         t={t}
       />
     )

packages/client/ui-conversation/src/client/chat/MessageIconActions.tsx

@@ -27,8 +27,6 @@ export interface MessageIconActionsProps {
   onBranch?: (() => void) | undefined
   /** The message is not a completed transcript tail, so branch stays visible but unavailable. */
   branchUnavailable?: boolean | undefined
-  /** Additional branch visibility gate for transient message chrome; defaults to true. */
-  showBranch?: boolean | undefined
   /** Parent layout class composed onto the actions row. */
   className?: string | undefined
   /** The owning view's locale seat, passed down as a plain prop. */
@@ -41,7 +39,7 @@ export interface MessageIconActionsProps {
  * @returns The actions row element.
  */
 export function MessageIconActions({
-  text, time, runMs, ttftMs, tokensPerSecond, clock, onBranch, branchUnavailable = false, showBranch = true, className, t,
+  text, time, runMs, ttftMs, tokensPerSecond, clock, onBranch, branchUnavailable = false, className, t,
 }: MessageIconActionsProps) {
   const day = useCalendarDay()
   const reasonId = useId()
@@ -111,7 +109,7 @@ export function MessageIconActions({
           {copied ? <IconCheckOutline16 /> : <IconCopyOutline16 />}
         </button>
       </Tooltip>
-      {showBranch && onBranch !== undefined && (
+      {onBranch !== undefined && (
         <Tooltip label={branchUnavailable ? t('message.branchUnavailable') : t('message.branch')} side="bottom">
           {/* Native disabled buttons do not deliver the hover/focus events Tooltip needs. */}
           <button
@@ -127,7 +125,7 @@ export function MessageIconActions({
           </button>
         </Tooltip>
       )}
-      {showBranch && onBranch !== undefined && branchUnavailable && (
+      {onBranch !== undefined && branchUnavailable && (
         <span id={reasonId} className={css.visuallyHidden}>{t('message.branchUnavailable')}</span>
       )}
       {clock === 'end' ? clockEl : null}

packages/client/ui-conversation/src/client/chat/MessageItem.tsx

@@ -1,8 +1,8 @@
 // MessageItem: simple chat nodes — user and consumed-steering bubbles
-// (right-aligned, with clock + copy / branch IconActions; steering adds the
-// interjection caption that names it), pending steering (caption + copy only),
-// context injection, compaction marker, retry disclosure, and unknown-surface
-// JSON rows.
+// (right-aligned, with clock + copy IconActions; steering adds the
+// interjection caption that names it; branch lives only under assistant
+// answers), pending steering (caption + copy only), context injection,
+// compaction marker, retry disclosure, and unknown-surface JSON rows.
 
 import { memo, useEffect, useMemo, useState } from 'react'
 import type { ReactNode } from 'react'
@@ -27,10 +27,6 @@ export interface MessageItemProps {
     | TurnErrorNode
     | UnknownSurfaceNode
   retryActive?: boolean
-  /** Fork through this message's completed turn when eligible. */
-  onFork?: (seq: number) => void
-  /** The message is not the transcript tail of a completed turn. */
-  forkUnavailable?: boolean
   /** The owning view's locale seat, passed down as a plain prop. */
   t: ChatViewSlotProps['t']
 }
@@ -217,7 +213,6 @@ export function PendingSteeringBubble({ content, t }: {
         <MessageIconActions
           text={text}
           clock="start"
-          showBranch={false}
           className={css.actions}
           t={t}
         />
@@ -227,7 +222,7 @@ export function PendingSteeringBubble({ content, t }: {
 }
 
 export const MessageItem = memo(function MessageItem({
-  node, retryActive = false, onFork, forkUnavailable = false, t,
+  node, retryActive = false, t,
 }: MessageItemProps) {
   const truncated = (total: number): string => t('json.truncated', { total })
   switch (node.kind) {
@@ -243,8 +238,6 @@ export const MessageItem = memo(function MessageItem({
               text={text}
               time={node.time}
               clock="start"
-              onBranch={onFork === undefined ? undefined : () => { onFork(node.seq) }}
-              branchUnavailable={forkUnavailable}
               className={css.actions}
               t={t}
             />

packages/client/ui-conversation/src/client/chat/chat-flow.ts

@@ -75,15 +75,18 @@ export function runningTurnStartTime(
 }
 
 /**
- * Seq set of message rows that may fork: the last transcript node of a
- * completed turn, when that node owns message chrome. A later tool, reasoning,
- * error, or other transcript node leaves the earlier message's branch action
- * unavailable because the Host would include the whole turn.
+ * Seq set of assistant answers that may fork: the completed turn's transcript
+ * tail, when that tail is the turn's own content-text assistant. A later tool,
+ * reasoning, error, or other transcript node leaves the answer's branch action
+ * unavailable because the Host would include the whole turn. User and steering
+ * bubbles carry no branch action at all: a fork at their seq cuts at the same
+ * `turn/end` as the answer's, so the affordance lives only under the settled
+ * answer.
  * @param nodes - snapshot nodes in event order.
  * @param turnEnds - completed turn boundaries retained from the event window.
- * @returns Message seq values whose visible position matches the fork boundary.
+ * @returns Assistant seq values whose visible position matches the fork boundary.
  */
-export function messageBranchSeqs(
+export function assistantBranchSeqs(
   nodes: readonly ConversationNode[],
   turnEnds: ReadonlyMap<number, number>,
 ): ReadonlySet<number> {
@@ -98,8 +101,7 @@ export function messageBranchSeqs(
       tail = candidate
       nodeIndex++
     }
-    if (tail?.kind === 'user' || tail?.kind === 'steering'
-      || (tail?.kind === 'assistant' && tail.turn === turn && hasContentText(tail.blocks))) {
+    if (tail?.kind === 'assistant' && tail.turn === turn && hasContentText(tail.blocks)) {
       result.add(tail.seq)
     }
   }