refactor(native-chat): tidy the rail wake hook and cover it with tests (#39477)

Store absolute deadlines instead of two last-seen timestamps, and let
scroll events with no wake pending return before reading the clock.
Behaviour is unchanged.

src/renderer/src/components/native-chat/use-native-chat-rail-wake.test.ts

@@ -0,0 +1,67 @@
+// @vitest-environment happy-dom
+import { renderHook } from '@testing-library/react'
+import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest'
+import { useNativeChatRailWake } from './use-native-chat-rail-wake'
+
+function setup() {
+  const hook = renderHook(() => useNativeChatRailWake())
+  const rail = document.createElement('div')
+  hook.result.current.railRef.current = rail
+  return { ...hook.result.current, rail, unmount: hook.unmount }
+}
+
+describe('useNativeChatRailWake', () => {
+  beforeEach(() => {
+    vi.useFakeTimers()
+  })
+  afterEach(() => {
+    vi.useRealTimers()
+  })
+
+  it('lights on input and goes dark after the wake window', () => {
+    const { rail, wake } = setup()
+    wake()
+    expect(rail.dataset.awake).toBe('')
+    vi.advanceTimersByTime(299)
+    expect('awake' in rail.dataset).toBe(true)
+    vi.advanceTimersByTime(1)
+    expect('awake' in rail.dataset).toBe(false)
+  })
+
+  it('stays lit while scroll events follow a recent input', () => {
+    const { rail, wake, sustain } = setup()
+    wake()
+    for (let i = 0; i < 10; i++) {
+      vi.advanceTimersByTime(100)
+      sustain()
+    }
+    expect('awake' in rail.dataset).toBe(true)
+    vi.advanceTimersByTime(300)
+    expect('awake' in rail.dataset).toBe(false)
+  })
+
+  it('stops sustaining once the input is too old', () => {
+    const { rail, wake, sustain } = setup()
+    wake()
+    for (let i = 0; i < 20; i++) {
+      vi.advanceTimersByTime(100)
+      sustain()
+    }
+    // 1.5 s after input, scroll events no longer extend the wake.
+    expect('awake' in rail.dataset).toBe(false)
+  })
+
+  it('ignores scroll events with no wake pending', () => {
+    const { rail, sustain } = setup()
+    sustain()
+    vi.advanceTimersByTime(1000)
+    expect('awake' in rail.dataset).toBe(false)
+  })
+
+  it('clears its timer on unmount', () => {
+    const { wake, unmount } = setup()
+    wake()
+    unmount()
+    expect(vi.getTimerCount()).toBe(0)
+  })
+})

src/renderer/src/components/native-chat/use-native-chat-rail-wake.ts

@@ -17,12 +17,15 @@ export function useNativeChatRailWake(): {
 } {
   const railRef = useRef<HTMLDivElement | null>(null)
   const timer = useRef<ReturnType<typeof setTimeout> | null>(null)
-  const lastWake = useRef(0)
-  const lastInput = useRef(0)
+  /** When the rail may go dark again. */
+  const sleepAt = useRef(0)
+  /** Scroll events before this still belong to the reader's last input. */
+  const sustainUntil = useRef(0)
 
   const wake = useCallback(() => {
-    lastWake.current = performance.now()
-    lastInput.current = lastWake.current
+    const now = performance.now()
+    sleepAt.current = now + NATIVE_CHAT_RAIL_WAKE_MS
+    sustainUntil.current = now + NATIVE_CHAT_RAIL_SUSTAIN_MS
     const rail = railRef.current
     // Why: a gesture fires wheel events every frame; write the DOM and arm a timer once per gesture.
     if (rail && !('awake' in rail.dataset)) {
@@ -32,7 +35,7 @@ export function useNativeChatRailWake(): {
       return
     }
     const settle = (): void => {
-      const remaining = lastWake.current + NATIVE_CHAT_RAIL_WAKE_MS - performance.now()
+      const remaining = sleepAt.current - performance.now()
       if (remaining > 0) {
         timer.current = setTimeout(settle, remaining)
         return
@@ -46,9 +49,13 @@ export function useNativeChatRailWake(): {
   }, [])
 
   const sustain = useCallback(() => {
+    // Why: most scroll events (streaming, auto-follow) arrive with no wake pending; skip the clock read.
+    if (timer.current === null) {
+      return
+    }
     const now = performance.now()
-    if (timer.current !== null && now - lastInput.current < NATIVE_CHAT_RAIL_SUSTAIN_MS) {
-      lastWake.current = now
+    if (now < sustainUntil.current) {
+      sleepAt.current = now + NATIVE_CHAT_RAIL_WAKE_MS
     }
   }, [])