diff --git a/tests/test-utils.ts b/tests/test-utils.ts
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--- /dev/null
+++ b/tests/test-utils.ts
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+/**
+ * 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()
+    },
+  }
+}