feat: 任务管理页二级开关「有新后台任务时自动展开」（autoOpenTasks）

- 新 pref autoOpenTasks（默认开）：当前会话出现新后台任务（任意新任务 id，
  非仅首个）时自动展开侧边栏并打开任务管理页，与 autoOpenSubagent 并列
- 注册为 subagent tab 的声明式二级开关（设置页齿轮弹窗，key 在 PrefsSchema）
- Sidebar 新增独立基线 effect（镜像子代理自动展开；刷新不触发、开关关闭/类型
  禁用不触发）；detectNewTask 纯函数（settling/同 id/他会话不触发）
- 测试：detectNewTask 用例、prefs 解析断言、builtins 开关清单、schema 默认值
  断言；全量 371 通过

src/client/Sidebar.tsx

@@ -44,6 +44,7 @@ import type { TabDragPayload } from './TabBar.tsx'
 import { relativeTo } from './paths.ts'
 import { OrphanedTab } from './OrphanedTab.tsx'
 import { detectNewDirectSubagent } from './subagent-detect.ts'
+import { detectNewTask } from './subagent-tasks.ts'
 import { t } from './locales.ts'
 import { api, type SessionScope } from './api.ts'
 import css from './sidebar.module.css'
@@ -241,6 +242,28 @@ export function Sidebar(props: { ctx: Context; store: SidebarStore }) {
     ctx.betterSidebar?.openTab({ type: 'subagent', title: t('subagent') })
   }, [sessionList, sessionId, store, ctx])
 
+  /**
+   * Task auto-activation: the moment a NEW background task appears for the
+   * current conversation (a task id the previous snapshot lacked), the
+   * auto-open pref is on, and the Tasks tab type is enabled, open the panel
+   * (if collapsed) and focus the Tasks page. Unlike the subagent trigger
+   * (0 → N only), ANY new task id triggers: the agent may start several
+   * tasks in one session, and each should surface. A fresh page load never
+   * triggers — its baseline starts at the current snapshot.
+   */
+  const taskBaselineRef = useRef<SidebarSessionList | undefined>(undefined)
+  useEffect(() => {
+    const prev = taskBaselineRef.current
+    taskBaselineRef.current = sessionList
+    if (sessionId === undefined || prev === undefined) return
+    if (!detectNewTask(prev, sessionList, sessionId)) return
+    if (!store.getPrefs().autoOpenTasks) return
+    if (ctx.betterSidebar?.isTabEnabled('subagent') === false) return
+    store.reduce(s => s.panelOpen ? s : togglePanel(s))
+    store.reduce(s => ({ ...s, activePane: firstLeaf(s.splits).id }))
+    ctx.betterSidebar?.openTab({ type: 'subagent', title: t('subagent') })
+  }, [sessionList, sessionId, store, ctx])
+
   /**
    * Topology jump-back: clicking a subagent node on the Subagent page calls
    * the official `openSubagent`, which switches the sidebar to that child

src/client/builtins/tabs.tsx

@@ -109,13 +109,17 @@ export function builtinTabs(ctx: Context): readonly TabDescriptor[] {
       icon: (size: number) => <IconThinkOutline16 size={size} />,
       order: 30,
       single: true,
-      // Declarative settings: the auto-open switch renders under this row in
-      // the Side card settings page (the Subagent page's own related setting).
+      // Declarative settings: the auto-open switches render under this row in
+      // the Side card settings page (the Tasks page's own related settings).
       settings: {
         toggles: [{
           key: 'autoOpenSubagent',
           title: () => t('settingsSubagentTitle'),
           desc: () => t('settingsSubagentDesc'),
+        }, {
+          key: 'autoOpenTasks',
+          title: () => t('settingsTasksTitle'),
+          desc: () => t('settingsTasksDesc'),
         }],
       },
       component: ({ ctx, scope, visible, onSubagentJump }) => (

src/client/locales.ts

@@ -121,6 +121,8 @@ const zh = {
   zoomHint: 'Alt + 滚轮',
   settingsSubagentTitle: '检测到子代理时自动展开任务管理页',
   settingsSubagentDesc: '当前会话产生新的子代理时，自动展开侧边栏并打开任务管理页；关闭后需手动打开',
+  settingsTasksTitle: '有新后台任务时自动展开任务管理页',
+  settingsTasksDesc: '当前会话出现新的后台任务时，自动展开侧边栏并打开任务管理页（每个新任务都会触发）；关闭后需手动打开',
   settingsToolsTitle: '为模型注入终端工具',
   settingsToolsDesc: '开启后，模型可通过 terminal_create 等 8 个工具创建并操作侧边栏终端（默认关闭）',
   settingsBottomTerminalTitle: '底部面板首次展开自动开终端',
@@ -194,7 +196,7 @@ const zh = {
   taskViewOutput: '查看输出',
   taskHideOutput: '收起输出',
   taskNoOutput: '暂无输出',
-  taskNotReadYet: '模型尚未读取该任务的输出，读取后会显示在这里',
+  taskNotReadYet: '等待模型读取该任务的输出（模型执行 task_output 后，输出会显示在这里）',
   taskOutputTruncated: '输出过长，已截断显示',
   taskOutputError: '输出读取失败',
   taskKill: '终止',
@@ -318,6 +320,8 @@ const en: Record<keyof typeof zh, string> = {
   zoomHint: 'Alt + wheel',
   settingsSubagentTitle: 'Auto-open the Tasks page when a subagent appears',
   settingsSubagentDesc: 'Expand the side card and open the Tasks page when the current conversation spawns a new subagent; turn off to open it manually',
+  settingsTasksTitle: 'Auto-open the Tasks page on a new background task',
+  settingsTasksDesc: 'Expand the side card and open the Tasks page whenever a new background task appears for the current conversation (every new task triggers); turn off to open it manually',
   settingsToolsTitle: 'Inject terminal tools for the model',
   settingsToolsDesc: 'When enabled, the model can create and drive sidebar terminals through the 8 terminal_* tools (off by default)',
   settingsBottomTerminalTitle: 'Auto-open a terminal on the bottom panel\'s first expansion',
@@ -391,7 +395,7 @@ const en: Record<keyof typeof zh, string> = {
   taskViewOutput: 'View output',
   taskHideOutput: 'Hide output',
   taskNoOutput: 'No output yet',
-  taskNotReadYet: 'The model has not read this task\'s output yet; it will appear here once read',
+  taskNotReadYet: 'Waiting for the model to read this task; its output appears here once the model runs task_output',
   taskOutputTruncated: 'Output truncated',
   taskOutputError: 'Failed to read output',
   taskKill: 'Kill',

src/client/prefs.ts

@@ -40,6 +40,9 @@ export function parsePrefs(value: unknown): SidebarPrefs {
     autoOpenSubagent: typeof record.autoOpenSubagent === 'boolean'
       ? record.autoOpenSubagent
       : SIDEBAR_PREFS_DEFAULTS.autoOpenSubagent,
+    autoOpenTasks: typeof record.autoOpenTasks === 'boolean'
+      ? record.autoOpenTasks
+      : SIDEBAR_PREFS_DEFAULTS.autoOpenTasks,
     agentTerminalTools: typeof record.agentTerminalTools === 'boolean'
       ? record.agentTerminalTools
       : SIDEBAR_PREFS_DEFAULTS.agentTerminalTools,

src/client/subagent-tasks.ts

@@ -57,6 +57,23 @@ export function treeSessionIds(
   return ids
 }
 
+/**
+ * Whether a NEW background task appeared for one session between two
+ * consecutive list snapshots (a task id the previous snapshot lacked).
+ * Unlike the subagent auto-open (0 → N only), ANY new task id triggers: the
+ * agent may start several tasks over a session, and each new one should
+ * surface the Tasks page (a fresh page load never triggers — its baseline
+ * starts at the current snapshot).
+ */
+export function detectNewTask(
+  prev: SidebarSessionList,
+  next: SidebarSessionList,
+  sessionId: string,
+): boolean {
+  const prevIds = new Set((prev.tasksBySession?.[sessionId] ?? []).map(task => task.id))
+  return (next.tasksBySession?.[sessionId] ?? []).some(task => !prevIds.has(task.id))
+}
+
 /**
  * Collect the background tasks of the whole current tree, owner-labeled.
  * Sessions without a mirror entry contribute nothing; an absent mirror

src/config.ts

@@ -79,6 +79,7 @@ export const PrefsSchema: z<SidebarPrefs> = z.object({
   openByDefault: z.boolean().default(true),
   defaultWidthPercent: z.number().step(1).min(WIDTH_PERCENT_MIN).max(WIDTH_PERCENT_MAX).default(WIDTH_PERCENT_DEFAULT),
   autoOpenSubagent: z.boolean().default(true),
+  autoOpenTasks: z.boolean().default(true),
   agentTerminalTools: z.boolean().default(false),
   bottomPanelAutoTerminal: z.boolean().default(true),
   interceptOpenPath: z.boolean().default(true),

src/prefs-shared.ts

@@ -20,6 +20,12 @@ export interface SidebarPrefs {
    * Subagent page when the current conversation spawns a new subagent.
    */
   autoOpenSubagent: boolean
+  /**
+   * Whether the sidebar auto-activates (opens the panel) and expands the
+   * Tasks page when a NEW background task appears for the current
+   * conversation (any new task id, not just the first one).
+   */
+  autoOpenTasks: boolean
   /**
    * Whether the model-facing agent terminal tools (terminal_create / list /
    * send / read / wait_for / resize / signal / close) are injected into the
@@ -103,6 +109,7 @@ export const SIDEBAR_PREFS_DEFAULTS: SidebarPrefs = {
   openByDefault: true,
   defaultWidthPercent: WIDTH_PERCENT_DEFAULT,
   autoOpenSubagent: true,
+  autoOpenTasks: true,
   agentTerminalTools: false,
   bottomPanelAutoTerminal: true,
   interceptOpenPath: true,