feat: fork

packages/client/ui-conversation/src/client/apply.ts

@@ -262,6 +262,13 @@ export function apply(ctx: Context): void {
           })
         },
         loadOlder: () => { void scoped.loadOlder() },
+        forkAt: (seq) => {
+          sessions.fork({ sessionId, atSeq: seq })
+            .then((childId) => { sessions.open(childId) })
+            .catch(() => {
+              // Fork failure keeps the source view untouched (composer-stop posture).
+            })
+        },
       }
     },
   }, ChatView)

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

@@ -230,7 +230,7 @@ function StreamingTail({ useSession, onGrow }: {
  * The chat view slot entry: pure component over the composed props (tool rows
  * render through the declared keyed hole's renderSlot share).
  */
-export function ChatView({ useSession, useSessions, useStore, renderSlot, sessionId, openFile, loadOlder }: ChatViewSlotProps) {
+export function ChatView({ useSession, useSessions, useStore, renderSlot, sessionId, openFile, loadOlder, forkAt }: ChatViewSlotProps) {
   const nodes = useSession(s => s.nodes)
   // Workspace root off the session list row: path summaries display relative to it.
   const cwd = useSessions(s => s.byId[sessionId]?.cwd)
@@ -385,7 +385,7 @@ export function ChatView({ useSession, useSessions, useStore, renderSlot, sessio
     }
     /* v8 ignore next -- tool-result never reaches here: deriveChatFlow folds them into groups. */
     if (node.kind === 'tool-result') return null
-    return <MessageItem key={item.key} node={node} />
+    return <MessageItem key={item.key} node={node} onFork={forkAt} />
   }
 
   return (

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

@@ -1,5 +1,6 @@
-// Shared IconActions chrome for user and assistant messages: copy / branch
-// live (branch still a stub), date-aware clock, optional edit stub.
+// Shared IconActions chrome for user and assistant messages: copy live,
+// branch wired through onBranch (stub without it), date-aware clock,
+// optional edit stub.
 
 import { useCallback } from 'react'
 import {
@@ -18,17 +19,19 @@ export interface MessageIconActionsProps {
   clock: 'start' | 'end'
   /** When true, append the stub edit control (user bubble). */
   edit?: boolean | undefined
+  /** Fork the session at this message; absent leaves the branch control a stub. */
+  onBranch?: (() => void) | undefined
   /** Parent layout class composed onto the actions row. */
   className?: string | undefined
 }
 
 /**
  * Copy / branch (/ clock) IconActions row shared by user and assistant chrome.
- * @param props - Copy text, event time, clock side, optional edit, className.
+ * @param props - Copy text, event time, clock side, optional edit, branch callback, className.
  * @returns The actions row element.
  */
 export function MessageIconActions({
-  text, time, clock, edit, className,
+  text, time, clock, edit, onBranch, className,
 }: MessageIconActionsProps) {
   const day = useCalendarDay()
   const onCopy = useCallback(() => {
@@ -48,7 +51,7 @@ export function MessageIconActions({
         </button>
       </Tooltip>
       <Tooltip label="在新对话中分支" side="bottom">
-        <button type="button" className={css.action} aria-label="在新对话中分支">
+        <button type="button" className={css.action} aria-label="在新对话中分支" onClick={onBranch}>
           <IconBranchOutline16 />
         </button>
       </Tooltip>

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

@@ -16,6 +16,8 @@ import css from './MessageItem.module.css'
 
 export interface MessageItemProps {
   node: UserMessageNode | SteeringMessageNode | ContextMessageNode | UnknownSurfaceNode
+  /** Fork the session through the turn containing this message (user-bubble branch action). */
+  onFork?: (seq: number) => void
 }
 
 function contentText(content: readonly unknown[]): { text: string; rest: unknown[] } {
@@ -61,7 +63,7 @@ function projectUserText(text: string): ReactNode {
   return <>{parts}</>
 }
 
-export const MessageItem = memo(function MessageItem({ node }: MessageItemProps) {
+export const MessageItem = memo(function MessageItem({ node, onFork }: MessageItemProps) {
   switch (node.kind) {
     case 'user': {
       const { text, rest } = contentText(node.content)
@@ -76,6 +78,7 @@ export const MessageItem = memo(function MessageItem({ node }: MessageItemProps)
             time={node.time}
             clock="start"
             edit
+            onBranch={onFork === undefined ? undefined : () => { onFork(node.seq) }}
             className={css.actions}
           />
         </div>

packages/client/ui-conversation/src/client/contract/slots.ts

@@ -419,6 +419,8 @@ export interface ChatViewInjected {
    */
   openFile: (path: string) => void
   loadOlder: () => void
+  /** Fork the session through the turn containing the message at `seq`, then open the child. */
+  forkAt: (seq: number) => void
 }
 
 /** Full chat-view component props: runtime share & the declared toolview/commandview holes' render share & store share & injected share. */