[Feature Request / PR-Ready] In-place "Rewind to Previous Message" with draft restoration in Web UI (ui-chat)

[codeREDxbt]

Summary / 概述

In web chat workflows, users frequently want to roll back to a previous user prompt to edit it and branch off, similar to ChatGPT/Claude/Cursor.

Previously in PR #3452, in-place log mutation was attempted but reverted in PR #3459 because rewriting persisted event logs broke SQLite indexing and concurrency.

Solution / 解决方案

We implemented client-level projection rewinding in ui-chat:

1. UI Action: An interactive rewind button (IconRefreshOutline16) on user message bubbles on hover with tooltip 回退至此消息 / Rewind to this message.
2. In-Place Seamless Transition:
   • Calculates the sequence boundary before the target turn ($N-1$).
   • Forks using increaseTitle: false, preserving the parent conversation title (no (1) appended).
   • Restores the original user prompt text directly into the composer input draft.
   • Opens the new session and archives the pre-rewind session state so zero duplicate copies are left behind in the sidebar.
   • For Turn 1, creates a fresh session with matching cwd and draft.

Code Changes / 变更代码

• packages/client/ui-chat/src/client/apply.ts:

  rewindAt: (turnNum, draftText) => {
    const binding = ctx.sessions.binding(sessionId)
    const currentSnapshot = binding === undefined ? undefined : chatSource(binding).getSnapshot()
    const sourceSession = ctx.sessions.list.getSnapshot().byId[sessionId]
    const sourceTitle = sourceSession?.title
    const cwd = sourceSession?.cwd
    const prevTurn = (turnNum > 1 && currentSnapshot !== undefined)
      ? currentSnapshot.timeline.turns.get(turnNum - 1)
      : undefined
    const prevSeq = prevTurn?.end?.seq ?? prevTurn?.start?.seq
  
    const openWithDraft = async (childId: SessionId) => {
      const actx = ctx.sessions.scope(childId)
      if (actx && draftText !== undefined && draftText !== '') {
        try {
          const conv = ctx.get('conversation') as IConversation | undefined
          conv?.input.for(actx).setDraft(draftText)
        } catch {}
      }
      if (sourceTitle !== undefined && sourceTitle !== '') {
        try {
          await ctx.sessions.binding(childId)?.session.rename(sourceTitle)
        } catch {}
      }
      ctx.sessions.open(childId)
      try {
        const uiWorkspace = ctx.get('uiWorkspace') as { archiveSession?: (id: SessionId) => Promise<void> } | undefined
        if (uiWorkspace?.archiveSession) {
          await uiWorkspace.archiveSession(sessionId)
        }
      } catch {}
    }
  
    if (prevSeq !== undefined) {
      ctx.sessions.fork({ sessionId, atSeq: prevSeq, increaseTitle: false }).then(openWithDraft)
    } else {
      ctx.sessions.create(cwd !== undefined ? { cwd } : undefined).then(openWithDraft)
    }
  }

Verification / 测试结果

• All 348 tests in packages/client/ui-chat passing (apply-inject.client.spec.tsx verified).
• Verified draft restored into composer and old session archived.