diff --git a/tests/add-plugin-modal.spec.tsx b/tests/add-plugin-modal.spec.tsx
index 6aedab3..d508dd0 100644
--- a/tests/add-plugin-modal.spec.tsx
+++ b/tests/add-plugin-modal.spec.tsx
@@ -26,7 +26,8 @@ import { createBetterSidebarService } from '../src/client/service.ts'
 import { createSidebarStore } from '../src/client/state.ts'
 
 // The act() environment flag (React 18.2 reads it before flushing effects).
-;(globalThis as Record<string, unknown>).IS_REACT_ACT_ENVIRONMENT = true
+import { setupReactAct } from './test-utils.ts'
+setupReactAct()
 
 const officeEntry = builtinViewerPlugins[0] as PluginEntry
 
diff --git a/tests/bottom-auto-terminal.spec.tsx b/tests/bottom-auto-terminal.spec.tsx
index 507eea3..8602197 100644
--- a/tests/bottom-auto-terminal.spec.tsx
+++ b/tests/bottom-auto-terminal.spec.tsx
@@ -20,7 +20,8 @@ import { createRoot, type Root } from 'react-dom/client'
 import { act } from 'react-dom/test-utils'
 
 // The act() environment flag (React 18.2 reads it before flushing effects).
-;(globalThis as Record<string, unknown>).IS_REACT_ACT_ENVIRONMENT = true
+import { setupReactAct } from './test-utils.ts'
+setupReactAct()
 
 import { Sidebar } from '../src/client/Sidebar.tsx'
 import { allLeaves, createSidebarStore, toggleBottomPanel, type SidebarStore } from '../src/client/state.ts'
diff --git a/tests/changes-tab.spec.tsx b/tests/changes-tab.spec.tsx
index 8187ce7..3093679 100644
--- a/tests/changes-tab.spec.tsx
+++ b/tests/changes-tab.spec.tsx
@@ -16,7 +16,8 @@ import { api, type GitStatusResult, type GitWorktree } from '../src/client/api.t
 import type { Context } from '../src/context-types.ts'
 import type { SidebarTab } from '../src/client/state.ts'
 
-;(globalThis as Record<string, unknown>).IS_REACT_ACT_ENVIRONMENT = true
+import { setupReactAct } from './test-utils.ts'
+setupReactAct()
 
 const MAIN = 'C:/repo/main'
 
diff --git a/tests/chunk-artifact.spec.ts b/tests/chunk-artifact.spec.ts
index 687d671..ec39364 100644
--- a/tests/chunk-artifact.spec.ts
+++ b/tests/chunk-artifact.spec.ts
@@ -5,8 +5,10 @@
  * be callable with a require that resolves the platform externals — the
  * exact shape the loader (src/client/chunk-loader.ts) depends on. Reads the
  * built lib/ output, so run `pnpm build` first (like manifest-consistency).
+ * A missing lib/ (fresh clone before the first build) skips the whole suite
+ * instead of crashing on ENOENT.
  */
-import { readFileSync } from 'node:fs'
+import { existsSync, readFileSync } from 'node:fs'
 import { describe, expect, it } from 'vitest'
 // Browser globals first: chunk bodies probe `self`/`document` at evaluation
 // (xterm's UMD wrapper, CodeMirror's UA probe).
@@ -17,7 +19,14 @@ const g = globalThis as Record<string, unknown>
 
 const CHUNKS = ['terminal', 'editor', 'mermaid']
 
-describe('built chunk artifacts', () => {
+/** All chunk artifacts present (tsdown emits the whole lib/ in one run). */
+const chunksBuilt = CHUNKS.every(name => existsSync(`lib/client-${name}.js`))
+
+if (!chunksBuilt) {
+  console.warn('[chunk-artifact] lib/ chunk artifacts missing — run `pnpm build` first; skipping this suite')
+}
+
+describe.skipIf(!chunksBuilt)('built chunk artifacts', () => {
   it('each chunk assigns its global registry slot when executed as a script', () => {
     g.window = g // classic-script globals
     // mermaid's core hooks window.addEventListener('load') at module scope
diff --git a/tests/diff-files-collapse.spec.tsx b/tests/diff-files-collapse.spec.tsx
index ce0627e..408c670 100644
--- a/tests/diff-files-collapse.spec.tsx
+++ b/tests/diff-files-collapse.spec.tsx
@@ -5,7 +5,8 @@ import { createRoot, type Root } from 'react-dom/client'
 import { act } from 'react-dom/test-utils'
 import { DiffFiles } from '../src/client/diff/DiffFiles.tsx'
 
-;(globalThis as Record<string, unknown>).IS_REACT_ACT_ENVIRONMENT = true
+import { setupReactAct } from './test-utils.ts'
+setupReactAct()
 
 const diff = [
   'diff --git a/src/a.ts b/src/a.ts',
diff --git a/tests/editor-host.spec.tsx b/tests/editor-host.spec.tsx
index 88a97e0..5f99828 100644
--- a/tests/editor-host.spec.tsx
+++ b/tests/editor-host.spec.tsx
@@ -19,7 +19,8 @@ import { createBetterSidebarService } from '../src/client/service.ts'
 import { allLeaves, createSidebarStore, type SidebarTab } from '../src/client/state.ts'
 
 // The act() environment flag (React 18.2 reads it before flushing effects).
-;(globalThis as Record<string, unknown>).IS_REACT_ACT_ENVIRONMENT = true
+import { setupReactAct } from './test-utils.ts'
+setupReactAct()
 
 /** A store with the seeded editor-home tab (default prefs: separate mode;
  *  merged-mode scenarios re-enable editorExplorer explicitly). */
diff --git a/tests/editor-refresh.spec.tsx b/tests/editor-refresh.spec.tsx
index 23ace84..6fcedb7 100644
--- a/tests/editor-refresh.spec.tsx
+++ b/tests/editor-refresh.spec.tsx
@@ -8,15 +8,15 @@
 // @vitest-environment jsdom
 import { beforeEach, describe, expect, it, vi } from 'vitest'
 import { createElement, useEffect, useState } from 'react'
-import { createRoot } from 'react-dom/client'
 import { act } from 'react-dom/test-utils'
+import { renderRoot, setupReactAct } from './test-utils.ts'
 import type { Context } from '../src/context-types.ts'
 import { EditorHost } from '../src/client/EditorHost.tsx'
 import { createBetterSidebarService, type FileViewerProps } from '../src/client/service.ts'
 import { allLeaves, createSidebarStore, type SidebarTab } from '../src/client/state.ts'
 
 // The act() environment flag (React 18.2 reads it before flushing effects).
-;(globalThis as Record<string, unknown>).IS_REACT_ACT_ENVIRONMENT = true
+setupReactAct()
 
 const fsRead = vi.fn()
 vi.mock('../src/client/api.ts', () => ({
@@ -90,22 +90,10 @@ function setup(initialMode: 'preview' | 'edit' = 'preview', dirty = false): {
 }
 
 function mount(ctx: Context, tab: () => SidebarTab): { container: HTMLDivElement; unmount: () => void } {
-  const container = document.createElement('div')
-  document.body.append(container)
-  const root = createRoot(container)
-  act(() => {
-    root.render(createElement(EditorHost, {
-      ctx, store: ctx.betterSidebar as never, scope: { sessionId: 'editor-home-session' },
-      tab: tab(), expanded: [], revealed: [], onToggleDir: () => {}, onReferenceFile: () => {},
-    }))
-  })
-  return {
-    container,
-    unmount: () => {
-      act(() => { root.unmount() })
-      container.remove()
-    },
-  }
+  return renderRoot(createElement(EditorHost, {
+    ctx, store: ctx.betterSidebar as never, scope: { sessionId: 'editor-home-session' },
+    tab: tab(), expanded: [], revealed: [], onToggleDir: () => {}, onReferenceFile: () => {},
+  }))
 }
 
 /** The header's refresh button (locale-dependent aria-label), or null. */
diff --git a/tests/file-tree-drop.spec.tsx b/tests/file-tree-drop.spec.tsx
index cfb414a..3a549b1 100644
--- a/tests/file-tree-drop.spec.tsx
+++ b/tests/file-tree-drop.spec.tsx
@@ -19,7 +19,8 @@ import { TAB_DRAG_TYPE } from '../src/client/TabBar.tsx'
 import type { UploadItem } from '../src/client/upload.ts'
 
 // The act() environment flag (React 18.2 reads it before flushing effects).
-;(globalThis as Record<string, unknown>).IS_REACT_ACT_ENVIRONMENT = true
+import { setupReactAct } from './test-utils.ts'
+setupReactAct()
 
 // vitest 4.1.11+ follows the OS locale; pin en-US so hint assertions are
 // deterministic regardless of the developer machine.
diff --git a/tests/file-tree-open-with.spec.tsx b/tests/file-tree-open-with.spec.tsx
index 50f8be7..013eadb 100644
--- a/tests/file-tree-open-with.spec.tsx
+++ b/tests/file-tree-open-with.spec.tsx
@@ -15,7 +15,8 @@ import { createSidebarStore } from '../src/client/state.ts'
 import type { OpenWithTarget } from '../src/client/open-with.ts'
 
 // The act() environment flag (React 18.2 reads it before flushing effects).
-;(globalThis as Record<string, unknown>).IS_REACT_ACT_ENVIRONMENT = true
+import { setupReactAct } from './test-utils.ts'
+setupReactAct()
 
 // vitest 4.1.11+ follows the OS locale; pin en-US so menu copy is English.
 beforeAll(() => {
diff --git a/tests/file-tree-reveal-scroll.spec.tsx b/tests/file-tree-reveal-scroll.spec.tsx
index 8ec025f..0d21cab 100644
--- a/tests/file-tree-reveal-scroll.spec.tsx
+++ b/tests/file-tree-reveal-scroll.spec.tsx
@@ -16,7 +16,8 @@ import { act } from 'react-dom/test-utils'
 import { FileTree } from '../src/client/FileTree.tsx'
 import { createSidebarStore } from '../src/client/state.ts'
 
-;(globalThis as Record<string, unknown>).IS_REACT_ACT_ENVIRONMENT = true
+import { setupReactAct } from './test-utils.ts'
+setupReactAct()
 
 // jsdom defines no scrollIntoView; counting calls needs a real function.
 const scrollIntoView = vi.fn()
diff --git a/tests/free-window.spec.tsx b/tests/free-window.spec.tsx
index 963f720..4f27842 100644
--- a/tests/free-window.spec.tsx
+++ b/tests/free-window.spec.tsx
@@ -20,7 +20,8 @@ import { createRoot, type Root } from 'react-dom/client'
 import { act } from 'react-dom/test-utils'
 
 // The act() environment flag (React 18.2 reads it before flushing effects).
-;(globalThis as Record<string, unknown>).IS_REACT_ACT_ENVIRONMENT = true
+import { setupReactAct } from './test-utils.ts'
+setupReactAct()
 
 import { Sidebar } from '../src/client/Sidebar.tsx'
 import { createSidebarStore, togglePanel, type SidebarStore, floatTab as floatTabReducer } from '../src/client/state.ts'
diff --git a/tests/git-view-worktree.spec.tsx b/tests/git-view-worktree.spec.tsx
index b399f42..e3a1b8a 100644
--- a/tests/git-view-worktree.spec.tsx
+++ b/tests/git-view-worktree.spec.tsx
@@ -13,7 +13,8 @@ import { GitLens } from '../src/client/changes/GitLens.tsx'
 import { createSidebarStore } from '../src/client/state.ts'
 import { api, type GitLogEntry, type GitStatusResult, type GitWorktree } from '../src/client/api.ts'
 
-;(globalThis as Record<string, unknown>).IS_REACT_ACT_ENVIRONMENT = true
+import { setupReactAct } from './test-utils.ts'
+setupReactAct()
 
 const MAIN = 'C:/repo/main'
 const AGENT = 'C:/repo/agent'
diff --git a/tests/lazy-chunk.spec.tsx b/tests/lazy-chunk.spec.tsx
index 3f23acf..f54523c 100644
--- a/tests/lazy-chunk.spec.tsx
+++ b/tests/lazy-chunk.spec.tsx
@@ -11,8 +11,8 @@
 // @vitest-environment jsdom
 import { afterEach, beforeEach, describe, expect, it } from 'vitest'
 import { createElement, type ComponentType, type ReactNode } from 'react'
-import { createRoot, type Root } from 'react-dom/client'
 import { act } from 'react-dom/test-utils'
+import { renderRoot } from './test-utils.ts'
 import { builtinTabs } from '../src/client/builtins/tabs.tsx'
 import { builtinViewers } from '../src/client/builtins/viewers.tsx'
 import { registerChunkForTests, resetChunks } from '../src/client/chunk-loader.ts'
@@ -21,21 +21,6 @@ import type { Context } from '../src/context-types.ts'
 import type { FileViewerProps, TabComponentProps } from '../src/client/service.ts'
 import css from '../src/client/sidebar.module.css'
 
-/** Render `node` into a detached body container under React's act(). */
-function mount(node: ReactNode): { container: HTMLDivElement; unmount: () => void } {
-  const container = document.createElement('div')
-  document.body.append(container)
-  const root: Root = createRoot(container)
-  // act() flushes the (concurrent) initial commit — the placeholder must be
-  // in the DOM before the async load flushes below.
-  act(() => { root.render(node) })
-  const unmount = (): void => {
-    act(() => { root.unmount() })
-    container.remove()
-  }
-  return { container, unmount }
-}
-
 const Marker = (): ReactNode => createElement('div', { 'data-testid': 'chunk-rendered' }, 'loaded')
 
 beforeEach(() => {
@@ -55,7 +40,7 @@ describe('lazyChunkComponent', () => {
       return { TextEditor: Marker }
     })
     const Wrapper = lazyChunkComponent<{ label: string }>('editor', (mod) => mod.TextEditor as ComponentType<{ label: string }> | undefined)
-    const { container, unmount } = mount(createElement(Wrapper, { label: 'x' }))
+    const { container, unmount } = renderRoot(createElement(Wrapper, { label: 'x' }))
     // Initial paint: the loading placeholder (no chunk loaded yet).
     expect(container.querySelector(`.${css.editorPlaceholder}`)).not.toBeNull()
     await act(async () => {})
@@ -72,7 +57,7 @@ describe('lazyChunkComponent', () => {
       return { TextEditor: Marker }
     })
     const Wrapper = lazyChunkComponent<Record<string, never>>('editor', (mod) => mod.TextEditor as ComponentType<Record<string, never>> | undefined)
-    const { container, unmount } = mount(createElement(Wrapper, {}))
+    const { container, unmount } = renderRoot(createElement(Wrapper, {}))
     await act(async () => {})
     expect(container.textContent).toContain('boom')
     // The failed load cleared the loader cache; retry now succeeds.
@@ -89,7 +74,7 @@ describe('lazyChunkComponent', () => {
     const Recorder = (props: { label: string }): ReactNode => createElement('div', { 'data-testid': 'rec', 'data-label': props.label })
     registerChunkForTests('terminal', async () => ({ TerminalView: Recorder }))
     const Wrapper = lazyChunkComponent<{ label: string }>('terminal', (mod) => mod.TerminalView as ComponentType<{ label: string }> | undefined)
-    const { container, unmount } = mount(createElement(Wrapper, { label: 'hello' }))
+    const { container, unmount } = renderRoot(createElement(Wrapper, { label: 'hello' }))
     await act(async () => {})
     expect(container.querySelector('[data-testid="rec"]')?.getAttribute('data-label')).toBe('hello')
     unmount()
@@ -123,7 +108,7 @@ describe('built-in descriptor contract (render-prop functions)', () => {
     // load fails and the wrapper must degrade gracefully, never throw.
     const viewers = builtinViewers()
     const code = viewers.find(viewer => viewer.id === 'code')!
-    const { container, unmount } = mount(createElement(code.component, {} as FileViewerProps))
+    const { container, unmount } = renderRoot(createElement(code.component, {} as FileViewerProps))
     await act(async () => {})
     expect(container.querySelector(`.${css.editorError}`)).not.toBeNull()
     expect(container.querySelector('button')).not.toBeNull()
@@ -135,7 +120,7 @@ describe('built-in descriptor contract (render-prop functions)', () => {
     const viewers = builtinViewers()
     const markdown = viewers.find(viewer => viewer.id === 'markdown')!
     // EditorHost renders viewer components via createElement(component, props).
-    const { container, unmount } = mount(createElement(markdown.component, {
+    const { container, unmount } = renderRoot(createElement(markdown.component, {
       ctx: {},
       store: undefined,
       scope: { sessionId: 's1', cwd: '/p' },
@@ -165,7 +150,7 @@ describe('built-in descriptor contract (render-prop functions)', () => {
       tab: { id: 'terminal:2', type: 'terminal', title: '终端 2' },
       visible: true,
     } as unknown as TabComponentProps
-    const { container, unmount } = mount(createElement(terminal.component, props))
+    const { container, unmount } = renderRoot(createElement(terminal.component, props))
     await act(async () => {})
     expect(container.querySelector('[data-testid="terminal-tabid"]')?.textContent).toBe('terminal:2')
     expect(received).toBe('terminal:2')
@@ -181,11 +166,11 @@ describe('built-in descriptor contract (render-prop functions)', () => {
     const tabs = builtinTabs({} as Context)
     const terminal = tabs.find(tab => tab.id === 'terminal')!
     const props = { tab: { id: 'terminal:1', type: 'terminal', title: '终端 1' } } as unknown as TabComponentProps
-    const { container: first, unmount: unmountFirst } = mount(createElement(terminal.component, props))
+    const { container: first, unmount: unmountFirst } = renderRoot(createElement(terminal.component, props))
     await act(async () => {})
     expect(first.querySelector('[data-testid="chunk-rendered"]')).not.toBeNull()
     unmountFirst()
-    const { container: second, unmount: unmountSecond } = mount(createElement(terminal.component, props))
+    const { container: second, unmount: unmountSecond } = renderRoot(createElement(terminal.component, props))
     await act(async () => {})
     expect(second.querySelector('[data-testid="chunk-rendered"]')).not.toBeNull()
     unmountSecond()
diff --git a/tests/manifest-consistency.spec.ts b/tests/manifest-consistency.spec.ts
index 00539e9..04ba007 100644
--- a/tests/manifest-consistency.spec.ts
+++ b/tests/manifest-consistency.spec.ts
@@ -11,7 +11,9 @@
  *   bundles must never be swapped.
  *
  * Reads the built lib/ output, so run `pnpm build` first (the registry
- * install validates main/client.main existence the same way).
+ * install validates main/client.main existence the same way). A missing
+ * lib/ (fresh clone before the first build) skips the whole suite instead
+ * of crashing on ENOENT.
  */
 import { describe, expect, it } from 'vitest'
 import { existsSync, readFileSync } from 'node:fs'
@@ -20,6 +22,13 @@ import { fileURLToPath } from 'node:url'
 
 const ROOT = resolve(fileURLToPath(new URL('..', import.meta.url)))
 
+/** The build output is present (tsdown emits the whole lib/ in one run). */
+const libBuilt = existsSync(resolve(ROOT, 'lib/client.js'))
+
+if (!libBuilt) {
+  console.warn('[manifest-consistency] lib/ build output missing — run `pnpm build` first; skipping this suite')
+}
+
 /** The registry manifest face this spec touches (mirror of the registry's PluginManifest). */
 interface PluginManifest {
   id: string
@@ -72,7 +81,7 @@ function chunkSlot(file: string): string {
   return match[1]!
 }
 
-describe('registry manifest consistency (dsh.plugin.json)', () => {
+describe.skipIf(!libBuilt)('registry manifest consistency (dsh.plugin.json)', () => {
   it('id is a valid two-segment registry id (native form)', () => {
     expect(manifest.id).toMatch(ID_PATTERN)
   })
diff --git a/tests/markdown-html-render.spec.tsx b/tests/markdown-html-render.spec.tsx
index 5e5d22e..8e0ca9d 100644
--- a/tests/markdown-html-render.spec.tsx
+++ b/tests/markdown-html-render.spec.tsx
@@ -15,7 +15,8 @@ import { MarkdownDocument, type MarkdownHtmlMedia } from '../src/client/Markdown
 import { analyzeMarkdownHtml } from '../src/client/markdown-html.ts'
 
 // The act() environment flag (React 18.2 reads it before flushing effects).
-;(globalThis as Record<string, unknown>).IS_REACT_ACT_ENVIRONMENT = true
+import { setupReactAct } from './test-utils.ts'
+setupReactAct()
 
 const media: MarkdownHtmlMedia = {
   scope: { sessionId: 's1', cwd: '/ws' },
diff --git a/tests/markdown-manual-refresh.spec.tsx b/tests/markdown-manual-refresh.spec.tsx
index d95acfd..1f2df30 100644
--- a/tests/markdown-manual-refresh.spec.tsx
+++ b/tests/markdown-manual-refresh.spec.tsx
@@ -5,15 +5,15 @@
 // @vitest-environment jsdom
 import { afterEach, describe, expect, it, vi } from 'vitest'
 import { createElement, useEffect } from 'react'
-import { createRoot } from 'react-dom/client'
 import { act } from 'react-dom/test-utils'
+import { renderRoot, setupReactAct } from './test-utils.ts'
 import type { Context } from '../src/context-types.ts'
 import { EditorHost } from '../src/client/EditorHost.tsx'
 import { api } from '../src/client/api.ts'
 import { createBetterSidebarService, type FileViewerProps } from '../src/client/service.ts'
 import { allLeaves, createSidebarStore, type SidebarTab } from '../src/client/state.ts'
 
-;(globalThis as Record<string, unknown>).IS_REACT_ACT_ENVIRONMENT = true
+setupReactAct()
 
 function toolbar(dirty: boolean) {
   return {
@@ -68,28 +68,16 @@ function setup(): {
 }
 
 function mount(ctx: Context, store: ReturnType<typeof createSidebarStore>, tab: SidebarTab) {
-  const container = document.createElement('div')
-  document.body.append(container)
-  const root = createRoot(container)
-  act(() => {
-    root.render(createElement(EditorHost, {
-      ctx,
-      store,
-      scope: { sessionId: 'markdown-manual-refresh-session', cwd: '/tmp' },
-      tab,
-      expanded: [],
-      revealed: [],
-      onToggleDir: () => {},
-      onReferenceFile: () => {},
-    }))
-  })
-  return {
-    container,
-    unmount: () => {
-      act(() => { root.unmount() })
-      container.remove()
-    },
-  }
+  return renderRoot(createElement(EditorHost, {
+    ctx,
+    store,
+    scope: { sessionId: 'markdown-manual-refresh-session', cwd: '/tmp' },
+    tab,
+    expanded: [],
+    revealed: [],
+    onToggleDir: () => {},
+    onReferenceFile: () => {},
+  }))
 }
 
 async function flush(): Promise<void> {
diff --git a/tests/md-toc.spec.tsx b/tests/md-toc.spec.tsx
index e0a0bb8..bd57384 100644
--- a/tests/md-toc.spec.tsx
+++ b/tests/md-toc.spec.tsx
@@ -13,7 +13,8 @@ import { act } from 'react-dom/test-utils'
 import { MdToc, TOC_MIN_HEADINGS } from '../src/client/md-toc.tsx'
 
 // The act() environment flag (React 18.2 reads it before flushing effects).
-;(globalThis as Record<string, unknown>).IS_REACT_ACT_ENVIRONMENT = true
+import { setupReactAct } from './test-utils.ts'
+setupReactAct()
 
 const scrollIntoView = vi.fn()
 Element.prototype.scrollIntoView = scrollIntoView
diff --git a/tests/mermaid-markdown.spec.tsx b/tests/mermaid-markdown.spec.tsx
index f5e3f37..3286dd8 100644
--- a/tests/mermaid-markdown.spec.tsx
+++ b/tests/mermaid-markdown.spec.tsx
@@ -12,7 +12,8 @@ import { createRoot, type Root } from 'react-dom/client'
 import { act } from 'react-dom/test-utils'
 
 // The act() environment flag (React 18.2 reads it before flushing effects).
-;(globalThis as Record<string, unknown>).IS_REACT_ACT_ENVIRONMENT = true
+import { setupReactAct } from './test-utils.ts'
+setupReactAct()
 
 vi.mock('mermaid', () => ({
   default: {
diff --git a/tests/open-with-settings.spec.tsx b/tests/open-with-settings.spec.tsx
index 06eabb3..7faf7c1 100644
--- a/tests/open-with-settings.spec.tsx
+++ b/tests/open-with-settings.spec.tsx
@@ -11,7 +11,8 @@ import { createRoot, type Root } from 'react-dom/client'
 import { act } from 'react-dom/test-utils'
 import { OpenWithSettings } from '../src/client/open-with-settings.tsx'
 
-;(globalThis as Record<string, unknown>).IS_REACT_ACT_ENVIRONMENT = true
+import { setupReactAct } from './test-utils.ts'
+setupReactAct()
 
 beforeAll(() => {
   Object.defineProperty(window.navigator, 'language', { value: 'en-US', configurable: true })
diff --git a/tests/selection-popup.spec.tsx b/tests/selection-popup.spec.tsx
index c366c2e..0915a2c 100644
--- a/tests/selection-popup.spec.tsx
+++ b/tests/selection-popup.spec.tsx
@@ -16,7 +16,8 @@ import { act } from 'react-dom/test-utils'
 import { useSelectionPopup } from '../src/client/selection-popup.ts'
 
 // The act() environment flag (React 18.2 reads it before flushing effects).
-;(globalThis as Record<string, unknown>).IS_REACT_ACT_ENVIRONMENT = true
+import { setupReactAct } from './test-utils.ts'
+setupReactAct()
 
 /** jsdom ships no IntersectionObserver; a controllable stand-in. */
 class FakeIntersectionObserver implements IntersectionObserver {
diff --git a/tests/side-card-section-rows.spec.tsx b/tests/side-card-section-rows.spec.tsx
index a873266..271f33f 100644
--- a/tests/side-card-section-rows.spec.tsx
+++ b/tests/side-card-section-rows.spec.tsx
@@ -12,32 +12,16 @@
  */
 // @vitest-environment jsdom
 import { describe, expect, it } from 'vitest'
-import { createElement, type ReactNode } from 'react'
-import { createRoot, type Root } from 'react-dom/client'
+import { createElement } from 'react'
 import { act } from 'react-dom/test-utils'
+import { renderRoot, setupReactAct } from './test-utils.ts'
 
 // The act() environment flag (React 18.2 reads it before flushing effects).
-;(globalThis as Record<string, unknown>).IS_REACT_ACT_ENVIRONMENT = true
+setupReactAct()
 import type { SidebarSettingToggle } from '../src/client/service.ts'
 import { FeatureSettingsRows } from '../src/client/SideCardSection.tsx'
 import { SIDEBAR_PREFS_DEFAULTS } from '../src/prefs-shared.ts'
 
-/** Render the rows into a detached container under React's act(). */
-function mount(node: ReactNode): { container: HTMLDivElement; rerender: (node: ReactNode) => void; unmount: () => void } {
-  const container = document.createElement('div')
-  document.body.append(container)
-  const root: Root = createRoot(container)
-  act(() => { root.render(node) })
-  return {
-    container,
-    rerender: (next) => { act(() => { root.render(next) }) },
-    unmount: () => {
-      act(() => { root.unmount() })
-      container.remove()
-    },
-  }
-}
-
 /** Type into an input and commit it via blur (React 18: input event +
  *  focusout). The native setter bypasses React's value tracker so the
  *  change is actually seen. */
@@ -60,7 +44,7 @@ describe('FeatureSettingsRows typed rows (interactive)', () => {
       type: 'text',
       title: () => 'Font family',
     }
-    const { container, unmount } = mount(createElement(FeatureSettingsRows, {
+    const { container, unmount } = renderRoot(createElement(FeatureSettingsRows, {
       toggles: [toggle],
       prefs,
       onToggle: () => {},
@@ -86,7 +70,7 @@ describe('FeatureSettingsRows typed rows (interactive)', () => {
       min: 9,
       max: 32,
     }
-    const { container, unmount } = mount(createElement(FeatureSettingsRows, {
+    const { container, unmount } = renderRoot(createElement(FeatureSettingsRows, {
       toggles: [toggle],
       prefs: { ...prefs, terminalFontSize: 13 },
       onToggle: () => {},
@@ -113,7 +97,7 @@ describe('FeatureSettingsRows typed rows (interactive)', () => {
       min: 9,
       max: 32,
     }
-    const { container, unmount } = mount(createElement(FeatureSettingsRows, {
+    const { container, unmount } = renderRoot(createElement(FeatureSettingsRows, {
       toggles: [toggle],
       prefs: { ...prefs, terminalFontSize: 13 },
       onToggle: () => {},
@@ -142,7 +126,7 @@ describe('FeatureSettingsRows typed rows (interactive)', () => {
     // The parent mirrors the real handler: the optimistic commit adopts the
     // typed value, and a FAILED write reverts prefs to the stored one.
     let prefsNow = { ...prefs, terminalFontFamily: 'Old' }
-    const { container, rerender, unmount } = mount(createElement(FeatureSettingsRows, {
+    const { container, rerender, unmount } = renderRoot(createElement(FeatureSettingsRows, {
       toggles: [toggle],
       prefs: prefsNow,
       onToggle: () => {},
@@ -198,7 +182,7 @@ describe('FeatureSettingsRows select rows (interactive)', () => {
       title: () => 'Editor explorer',
       options,
     }
-    const { container, unmount } = mount(createElement(FeatureSettingsRows, {
+    const { container, unmount } = renderRoot(createElement(FeatureSettingsRows, {
       toggles: [toggle],
       // Merged is the selected option for this scenario (the default is now
       // separate — the anchor must render whatever value the prefs carry).
@@ -239,7 +223,7 @@ describe('FeatureSettingsRows select rows (interactive)', () => {
       onSelectValue: (t, next) => { commits.push([t.key, next]) },
       valueSource: () => value,
     })
-    const { container, rerender, unmount } = mount(rows(['c', 'a']))
+    const { container, rerender, unmount } = renderRoot(rows(['c', 'a']))
     // The anchor shows the picked titles; options follow the declared order.
     const items = openSelect(container)
     expect(items.map(item => item.textContent)).toEqual(['Alpha', 'Beta', 'Gamma'])
@@ -263,7 +247,7 @@ describe('FeatureSettingsRows select rows (interactive)', () => {
       title: () => 'Editor explorer',
       options,
     }
-    const { container, unmount } = mount(createElement(FeatureSettingsRows, {
+    const { container, unmount } = renderRoot(createElement(FeatureSettingsRows, {
       toggles: [toggle],
       prefs,
       onToggle: () => {},
diff --git a/tests/sidebar-auto-activation.spec.tsx b/tests/sidebar-auto-activation.spec.tsx
index 13efa58..548e4f6 100644
--- a/tests/sidebar-auto-activation.spec.tsx
+++ b/tests/sidebar-auto-activation.spec.tsx
@@ -9,7 +9,8 @@ import { createElement } from 'react'
 import { createRoot, type Root } from 'react-dom/client'
 import { act } from 'react-dom/test-utils'
 
-;(globalThis as Record<string, unknown>).IS_REACT_ACT_ENVIRONMENT = true
+import { setupReactAct } from './test-utils.ts'
+setupReactAct()
 
 import { Sidebar } from '../src/client/Sidebar.tsx'
 import { allLeaves, createSidebarStore, firstLeaf, type SidebarStore } from '../src/client/state.ts'
diff --git a/tests/sidebar-crash.spec.tsx b/tests/sidebar-crash.spec.tsx
index 9bab02c..bf2bb7b 100644
--- a/tests/sidebar-crash.spec.tsx
+++ b/tests/sidebar-crash.spec.tsx
@@ -23,7 +23,8 @@ import { createRoot, type Root } from 'react-dom/client'
 import { act } from 'react-dom/test-utils'
 
 // The act() environment flag (React 18.2 reads it before flushing effects).
-;(globalThis as Record<string, unknown>).IS_REACT_ACT_ENVIRONMENT = true
+import { setupReactAct } from './test-utils.ts'
+setupReactAct()
 
 import { Sidebar } from '../src/client/Sidebar.tsx'
 import { createSidebarStore, setBottomHeight, toggleBottomPanel, type SidebarStore } from '../src/client/state.ts'
diff --git a/tests/sidechat-view.spec.tsx b/tests/sidechat-view.spec.tsx
index 0915199..2d52500 100644
--- a/tests/sidechat-view.spec.tsx
+++ b/tests/sidechat-view.spec.tsx
@@ -8,15 +8,15 @@
  */
 // @vitest-environment jsdom
 import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest'
-import { createElement, type ReactNode } from 'react'
-import { createRoot, type Root } from 'react-dom/client'
+import { createElement } from 'react'
 import { act } from 'react-dom/test-utils'
+import { renderRoot, setupReactAct } from './test-utils.ts'
 import { SideChatView } from '../src/client/SideChatView.tsx'
 import { attachLocale } from '../src/client/locales.ts'
 import type { Context, SidebarSessionList } from '../src/context-types.ts'
 import type { SidebarTab } from '../src/client/state.ts'
 
-;(globalThis as Record<string, unknown>).IS_REACT_ACT_ENVIRONMENT = true
+setupReactAct()
 
 /** Minimal structural fake of the DSH LocaleService face the sidebar uses. */
 class FakeLocale {
@@ -110,19 +110,6 @@ function jsonResponse(value: unknown): Response {
   return { ok: true, status: 200, json: async () => value } as unknown as Response
 }
 
-/** Render `node` into a detached body container under React's act(). */
-function mount(node: ReactNode): { container: HTMLDivElement; unmount: () => void } {
-  const container = document.createElement('div')
-  document.body.append(container)
-  const root: Root = createRoot(container)
-  act(() => { root.render(node) })
-  const unmount = (): void => {
-    act(() => { root.unmount() })
-    container.remove()
-  }
-  return { container, unmount }
-}
-
 let eventsPulls = 0
 
 beforeEach(() => {
@@ -168,7 +155,7 @@ function threadStore(): ReturnType<typeof makeStore> {
 describe('SideChatView rendering', () => {
   it('renders the bash pair as a TerminalBlock and the turn tail with usage + duration', async () => {
     const props = viewProps(makeCtx(threadStore()))
-    const { container, unmount } = mount(createElement(SideChatView, props))
+    const { container, unmount } = renderRoot(createElement(SideChatView, props))
     await act(async () => {})  // flush the initial transcript pull
     const text = container.textContent ?? ''
     // Terminal surface: the command line and the marker-stripped output body.
@@ -182,7 +169,7 @@ describe('SideChatView rendering', () => {
   it('shows the disconnect banner, reconnects on click, and pulls immediately on recovery', async () => {
     const connection = makeConnection('disconnected')
     const props = viewProps(makeCtx(threadStore(), connection))
-    const { container, unmount } = mount(createElement(SideChatView, props))
+    const { container, unmount } = renderRoot(createElement(SideChatView, props))
     await act(async () => {})  // flush the initial transcript pull
     expect(container.textContent ?? '').toContain('连接已断开')
 
@@ -200,12 +187,12 @@ describe('SideChatView rendering', () => {
   })
 
   it('renders no banner without a connection service or while connected', async () => {
-    const absent = mount(createElement(SideChatView, viewProps(makeCtx(threadStore()))))
+    const absent = renderRoot(createElement(SideChatView, viewProps(makeCtx(threadStore()))))
     expect(absent.container.textContent ?? '').not.toContain('连接已断开')
     absent.unmount()
 
     const connection = makeConnection('connected')
-    const connected = mount(createElement(SideChatView, viewProps(makeCtx(threadStore(), connection))))
+    const connected = renderRoot(createElement(SideChatView, viewProps(makeCtx(threadStore(), connection))))
     expect(connected.container.textContent ?? '').not.toContain('连接已断开')
     connected.unmount()
   })
diff --git a/tests/subagent-jobs-view.spec.tsx b/tests/subagent-jobs-view.spec.tsx
index 9c5e37b..1af5b00 100644
--- a/tests/subagent-jobs-view.spec.tsx
+++ b/tests/subagent-jobs-view.spec.tsx
@@ -7,9 +7,9 @@
  */
 // @vitest-environment jsdom
 import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest'
-import { createElement, type ReactNode } from 'react'
-import { createRoot, type Root } from 'react-dom/client'
+import { createElement } from 'react'
 import { act } from 'react-dom/test-utils'
+import { renderRoot } from './test-utils.ts'
 import { SubagentView } from '../src/client/SubagentView.tsx'
 import type { Context, SidebarSessionList } from '../src/context-types.ts'
 
@@ -52,19 +52,6 @@ function makeCtx(store: Store): Context {
   } as unknown as Context
 }
 
-/** Render `node` into a detached body container under React's act(). */
-function mount(node: ReactNode): { container: HTMLDivElement; unmount: () => void } {
-  const container = document.createElement('div')
-  document.body.append(container)
-  const root: Root = createRoot(container)
-  act(() => { root.render(node) })
-  const unmount = (): void => {
-    act(() => { root.unmount() })
-    container.remove()
-  }
-  return { container, unmount }
-}
-
 const outputCalls: Array<{ sessionId: string; id: string }> = []
 const killCalls: Array<{ sessionId: string; id: string }> = []
 
@@ -129,7 +116,7 @@ afterEach(() => {
 describe('SubagentView background jobs', () => {
   it('renders the tree jobs with status, durations, and owner labels', () => {
     const store = makeStore(baseSnapshot())
-    const { container, unmount } = mount(
+    const { container, unmount } = renderRoot(
       createElement(SubagentView, { sessionId: 'root', active: true, ctx: makeCtx(store) }),
     )
     const text = container.textContent ?? ''
@@ -147,7 +134,7 @@ describe('SubagentView background jobs', () => {
 
   it('renders nothing job-related when the mirror is empty', () => {
     const store = makeStore({ current: 'root', byId: { root: { id: 'root', displayTitle: '主会话' } }, subagentsByParent: {}, jobsBySession: {} })
-    const { container, unmount } = mount(
+    const { container, unmount } = renderRoot(
       createElement(SubagentView, { sessionId: 'root', active: true, ctx: makeCtx(store) }),
     )
     expect(container.textContent).not.toContain('后台任务')
@@ -156,7 +143,7 @@ describe('SubagentView background jobs', () => {
 
   it('survives the mirror emptying while mounted (hook-order regression #300)', async () => {
     const store = makeStore(baseSnapshot())
-    const { container, unmount } = mount(
+    const { container, unmount } = renderRoot(
       createElement(SubagentView, { sessionId: 'root', active: true, ctx: makeCtx(store) }),
     )
     expect(container.textContent).toContain('sleep 300')
@@ -174,7 +161,7 @@ describe('SubagentView background jobs', () => {
 
   it('shows the selected job output in the bottom dock, closeable', async () => {
     const store = makeStore(baseSnapshot())
-    const { container, unmount } = mount(
+    const { container, unmount } = renderRoot(
       createElement(SubagentView, { sessionId: 'root', active: true, ctx: makeCtx(store) }),
     )
     const row = container.querySelector('button[aria-label*="sleep 300"]') as HTMLButtonElement
@@ -195,7 +182,7 @@ describe('SubagentView background jobs', () => {
 
   it('switches the single dock between selected rows', async () => {
     const store = makeStore(baseSnapshot())
-    const { container, unmount } = mount(
+    const { container, unmount } = renderRoot(
       createElement(SubagentView, { sessionId: 'root', active: true, ctx: makeCtx(store) }),
     )
     const first = container.querySelector('button[aria-label*="sleep 300"]') as HTMLButtonElement
@@ -223,7 +210,7 @@ describe('SubagentView background jobs', () => {
       ],
     }
     const store = makeStore(snapshot)
-    const { container, unmount } = mount(
+    const { container, unmount } = renderRoot(
       createElement(SubagentView, { sessionId: 'root', active: true, ctx: makeCtx(store) }),
     )
     const row = container.querySelector('button[aria-label*="unread cmd"]') as HTMLButtonElement
@@ -249,7 +236,7 @@ describe('SubagentView background jobs', () => {
       subagentsByParent: {},
       jobsBySession: { root: many },
     })
-    const { container, unmount } = mount(
+    const { container, unmount } = renderRoot(
       createElement(SubagentView, { sessionId: 'root', active: true, ctx: makeCtx(store) }),
     )
     expect(container.textContent).toContain('60 个后台任务 · 30 运行中')
@@ -264,7 +251,7 @@ describe('SubagentView background jobs', () => {
 
   it('kills a live job only after the two-click confirm', async () => {
     const store = makeStore(baseSnapshot())
-    const { container, unmount } = mount(
+    const { container, unmount } = renderRoot(
       createElement(SubagentView, { sessionId: 'root', active: true, ctx: makeCtx(store) }),
     )
     const kill = container.querySelector('button[aria-label="终止"]') as HTMLButtonElement
@@ -282,7 +269,7 @@ describe('SubagentView background jobs', () => {
     vi.useFakeTimers()
     try {
       const store = makeStore(baseSnapshot())
-      const { container, unmount } = mount(
+      const { container, unmount } = renderRoot(
         createElement(SubagentView, { sessionId: 'root', active: false, ctx: makeCtx(store) }),
       )
       const row = container.querySelector('button[aria-label*="sleep 300"]') as HTMLButtonElement
diff --git a/tests/subagent-live-polling.spec.tsx b/tests/subagent-live-polling.spec.tsx
index 4bfc4b0..685c841 100644
--- a/tests/subagent-live-polling.spec.tsx
+++ b/tests/subagent-live-polling.spec.tsx
@@ -6,9 +6,9 @@
  */
 // @vitest-environment jsdom
 import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest'
-import { createElement, type ReactNode } from 'react'
-import { createRoot, type Root } from 'react-dom/client'
+import { createElement } from 'react'
 import { act } from 'react-dom/test-utils'
+import { renderRoot } from './test-utils.ts'
 import { SubagentView } from '../src/client/SubagentView.tsx'
 import type { Context, SidebarSessionList } from '../src/context-types.ts'
 
@@ -51,24 +51,6 @@ function makeCtx(store: Store, historySpy: ReturnType<typeof vi.fn>): Context {
   } as unknown as Context
 }
 
-/** Render `node` into a detached body container under React's act(). */
-function mount(node: ReactNode): {
-  container: HTMLDivElement
-  rerender: (next: ReactNode) => void
-  unmount: () => void
-} {
-  const container = document.createElement('div')
-  document.body.append(container)
-  const root: Root = createRoot(container)
-  const rerender = (next: ReactNode): void => { act(() => { root.render(next) }) }
-  act(() => { root.render(node) })
-  const unmount = (): void => {
-    act(() => { root.unmount() })
-    container.remove()
-  }
-  return { container, rerender, unmount }
-}
-
 function jsonResponse(value: unknown): Response {
   return { ok: true, status: 200, json: async () => value } as unknown as Response
 }
@@ -156,7 +138,7 @@ describe('SubagentView live polling', () => {
     })
 
     const store = makeStore(runningSnapshot())
-    const { container, unmount } = mount(
+    const { container, unmount } = renderRoot(
       createElement(SubagentView, { sessionId: 'root', active: true, ctx: makeCtx(store, historySpy) }),
     )
     // Initial load plus the first scheduled tick.
@@ -187,7 +169,7 @@ describe('SubagentView live polling', () => {
     })
 
     const store = makeStore(runningSnapshot())
-    const { unmount } = mount(
+    const { unmount } = renderRoot(
       createElement(SubagentView, { sessionId: 'root', active: true, ctx: makeCtx(store, historySpy) }),
     )
     await act(async () => { await Promise.resolve() })
@@ -225,7 +207,7 @@ describe('SubagentView live polling', () => {
     })
 
     const store = makeStore(runningSnapshot())
-    const { rerender, unmount } = mount(
+    const { rerender, unmount } = renderRoot(
       createElement(SubagentView, { sessionId: 'root', active: true, ctx: makeCtx(store, historySpy) }),
     )
     await act(async () => { await Promise.resolve() })
@@ -264,7 +246,7 @@ describe('SubagentView live polling', () => {
     })
 
     const store = makeStore(runningSnapshot())
-    const { container, unmount } = mount(
+    const { container, unmount } = renderRoot(
       createElement(SubagentView, { sessionId: 'root', active: true, ctx: makeCtx(store, historySpy) }),
     )
     await act(async () => { await Promise.resolve() })
diff --git a/tests/tab-bar-context-menu.spec.tsx b/tests/tab-bar-context-menu.spec.tsx
index 1397fd8..3133ce3 100644
--- a/tests/tab-bar-context-menu.spec.tsx
+++ b/tests/tab-bar-context-menu.spec.tsx
@@ -16,7 +16,8 @@ import { createRoot, type Root } from 'react-dom/client'
 import { act } from 'react-dom/test-utils'
 
 // The act() environment flag (React 18.2 reads it before flushing effects).
-;(globalThis as Record<string, unknown>).IS_REACT_ACT_ENVIRONMENT = true
+import { setupReactAct } from './test-utils.ts'
+setupReactAct()
 
 import { TabBar } from '../src/client/TabBar.tsx'
 import type { SidebarTab } from '../src/client/state.ts'
diff --git a/tests/tab-bar-middle-click.spec.tsx b/tests/tab-bar-middle-click.spec.tsx
index 74c95b4..ad8ed15 100644
--- a/tests/tab-bar-middle-click.spec.tsx
+++ b/tests/tab-bar-middle-click.spec.tsx
@@ -17,7 +17,8 @@ import { createRoot, type Root } from 'react-dom/client'
 import { act } from 'react-dom/test-utils'
 
 // The act() environment flag (React 18.2 reads it before flushing effects).
-;(globalThis as Record<string, unknown>).IS_REACT_ACT_ENVIRONMENT = true
+import { setupReactAct } from './test-utils.ts'
+setupReactAct()
 
 import { TabBar } from '../src/client/TabBar.tsx'
 import type { SidebarTab } from '../src/client/state.ts'
diff --git a/tests/tab-bar-wheel.spec.tsx b/tests/tab-bar-wheel.spec.tsx
index 78e69f9..99b4cf9 100644
--- a/tests/tab-bar-wheel.spec.tsx
+++ b/tests/tab-bar-wheel.spec.tsx
@@ -14,7 +14,8 @@ import { createRoot, type Root } from 'react-dom/client'
 import { act } from 'react-dom/test-utils'
 
 // The act() environment flag (React 18.2 reads it before flushing effects).
-;(globalThis as Record<string, unknown>).IS_REACT_ACT_ENVIRONMENT = true
+import { setupReactAct } from './test-utils.ts'
+setupReactAct()
 
 import { TabBar } from '../src/client/TabBar.tsx'
 import type { SidebarTab } from '../src/client/state.ts'
diff --git a/tests/terminal-deps-banner.spec.tsx b/tests/terminal-deps-banner.spec.tsx
index 2ab76ad..8cc55ea 100644
--- a/tests/terminal-deps-banner.spec.tsx
+++ b/tests/terminal-deps-banner.spec.tsx
@@ -19,7 +19,8 @@ import * as primitives from '@deepseek-ai/dsh-client-ui-primitives'
 import { TerminalDepsBanner } from '../src/client/TerminalView.tsx'
 
 // The act() environment flag (React 18.2 reads it before flushing effects).
-;(globalThis as Record<string, unknown>).IS_REACT_ACT_ENVIRONMENT = true
+import { setupReactAct } from './test-utils.ts'
+setupReactAct()
 
 const deps = {
   ok: false as const,
diff --git a/tests/test-utils.ts b/tests/test-utils.ts
new file mode 100644
index 0000000..cb38baf
--- /dev/null
+++ b/tests/test-utils.ts
@@ -0,0 +1,63 @@
+/**
+ * Shared React helpers for the component specs (tests/*.spec.tsx): the two
+ * pieces of boilerplate every createRoot-based spec used to repeat inline —
+ * - the React 18 act() environment flag, without which act() warns on every
+ *   flush (setupReactAct(), idempotent, called at module scope per spec —
+ *   vitest isolates each spec file in its own global, so per-file setup is
+ *   required, exactly like the inlined statement it replaces),
+ * - the detached-container render cycle (div → document.body.append →
+ *   createRoot → act(render) → act(unmount) + container.remove), collected
+ *   as renderRoot().
+ *
+ * Deliberately minimal: no querying DSL, no auto-cleanup registry, no new
+ * dependencies. Specs with bespoke harnesses (domain fixtures around the
+ * render, SSR renders, module-scoped container state) keep their own mounts.
+ */
+import type { ReactNode } from 'react'
+import { createRoot, type Root } from 'react-dom/client'
+import { act } from 'react-dom/test-utils'
+
+/**
+ * Declare this file's environment as a React act() environment (React 18.2
+ * reads the flag before flushing effects). Assigning `true` is idempotent,
+ * so every spec may call it unconditionally at module scope.
+ */
+export function setupReactAct(): void {
+  ;(globalThis as Record<string, unknown>).IS_REACT_ACT_ENVIRONMENT = true
+}
+
+/** What renderRoot() hands back: the DOM container plus act()-wrapped controls. */
+export interface RenderedRoot {
+  /** The div renderRoot() appended to document.body. */
+  container: HTMLDivElement
+  /** The react-dom root, for direct render/unmount control if ever needed. */
+  root: Root
+  /** Re-render a new element tree under act(). */
+  rerender: (next: ReactNode) => void
+  /** Unmount under act() and drop the container from document.body. */
+  unmount: () => void
+}
+
+/**
+ * Render `node` into a detached div appended to document.body and flush the
+ * initial commit under act() — the concurrent initial commit is flushed, so
+ * the rendered tree is already in the DOM when renderRoot() returns.
+ * rerender(next) re-renders under act(); unmount() unmounts under act() and
+ * removes the container (call it once — like the per-spec mounts this
+ * replaces, it is not guarded against double unmount).
+ */
+export function renderRoot(node: ReactNode): RenderedRoot {
+  const container = document.createElement('div')
+  document.body.append(container)
+  const root: Root = createRoot(container)
+  act(() => { root.render(node) })
+  return {
+    container,
+    root,
+    rerender: (next: ReactNode): void => { act(() => { root.render(next) }) },
+    unmount: (): void => {
+      act(() => { root.unmount() })
+      container.remove()
+    },
+  }
+}