chore: import upstream snapshot with attribution

apps/sim/hooks/use-smooth-text.test.tsx

@@ -0,0 +1,91 @@
+/**
+ * @vitest-environment jsdom
+ */
+import { act } from 'react'
+import { createRoot, type Root } from 'react-dom/client'
+import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest'
+import { useSmoothText } from '@/hooks/use-smooth-text'
+
+interface ProbeProps {
+  content: string
+  isStreaming: boolean
+  snapOnNonAppend?: boolean
+}
+
+/**
+ * Minimal dependency-free hook harness (the repo has no `@testing-library/react`). Mounts the hook in
+ * a real React root under jsdom so effects and refs run exactly as in the app. Fake timers keep the
+ * paced reveal from advancing, so each assertion observes the synchronous reveal decision only.
+ */
+function renderSmoothText(initial: ProbeProps) {
+  ;(globalThis as { IS_REACT_ACT_ENVIRONMENT?: boolean }).IS_REACT_ACT_ENVIRONMENT = true
+  const container = document.createElement('div')
+  const root: Root = createRoot(container)
+  const props = { ...initial }
+  let latest = ''
+
+  function Probe(p: ProbeProps) {
+    latest = useSmoothText(p.content, p.isStreaming, { snapOnNonAppend: p.snapOnNonAppend })
+    return null
+  }
+
+  const render = () =>
+    act(() => {
+      root.render(<Probe {...props} />)
+    })
+  render()
+
+  return {
+    value: () => latest,
+    rerender: (next: Partial<ProbeProps>) => {
+      Object.assign(props, next)
+      render()
+    },
+    unmount: () => act(() => root.unmount()),
+  }
+}
+
+const LONG = `# Existing Document\n\n${'Lorem ipsum dolor sit amet, '.repeat(8)}`
+
+describe('useSmoothText — streaming that begins on an already-open document', () => {
+  beforeEach(() => {
+    vi.useFakeTimers()
+  })
+  afterEach(() => {
+    vi.clearAllTimers()
+    vi.useRealTimers()
+  })
+
+  it('reveals a pre-existing document in full when an edit stream starts (no full-file replay)', () => {
+    // The editor mounts showing a static file (no stream yet).
+    const h = renderSmoothText({ content: '', isStreaming: false, snapOnNonAppend: true })
+    expect(h.value()).toBe('')
+
+    // The agent begins editing it: the first streamed value carries the whole existing document.
+    // It must appear instantly, not replay word-by-word from the first character.
+    h.rerender({ content: LONG, isStreaming: true })
+    expect(h.value()).toBe(LONG)
+    h.unmount()
+  })
+
+  it('still animates a brand-new file from the start (short content stays below the threshold)', () => {
+    // A create stream mounts already-streaming with a tiny first chunk → begins empty and paces in.
+    const h = renderSmoothText({ content: '# New file', isStreaming: true, snapOnNonAppend: true })
+    expect(h.value()).toBe('')
+    h.unmount()
+  })
+
+  it('shows content that is already large at mount in full (mount-time skip, unchanged)', () => {
+    const h = renderSmoothText({ content: LONG, isStreaming: true, snapOnNonAppend: true })
+    expect(h.value()).toBe(LONG)
+    h.unmount()
+  })
+
+  it('does not pre-reveal for chat (mounts already streaming with a small first chunk)', () => {
+    // Chat (no snapOnNonAppend) mounts streaming; the not-streaming→streaming edge never occurs, so
+    // the new transition skip cannot fire and ordinary paced reveal is preserved.
+    const h = renderSmoothText({ content: 'Hello', isStreaming: true })
+    expect(h.value()).toBe('')
+    h.unmount()
+  })
+})