feat(mobile): load earlier chat messages on scroll-up, keeping the reader's place

mobile/src/session/MobileNativeChatMessage.paced-reveal.test.ts

@@ -11,6 +11,10 @@ import {
   MobileNativeChatReplyRevealsContext,
   type MobileNativeChatReplyReveals
 } from './mobile-native-chat-reply-reveals'
+import {
+  createMobileNativeChatGrowthHold,
+  MobileNativeChatGrowthHoldContext
+} from './mobile-native-chat-growth-hold'
 
 vi.mock('react-native', () => ({
   Image: 'Image',
@@ -39,6 +43,8 @@ const second = first + 'more '.repeat(40)
 describe('a growing reply on the phone', () => {
   let renderer: ReactTestRenderer | undefined
   let chat: MobileNativeChatReplyReveals
+  // The page's hold on growth, as a reader who scrolled up moves.
+  let growth = createMobileNativeChatGrowthHold()
 
   function render(text: string | string[], working = true, runningTool = false): void {
     const message: NativeChatMessage = {
@@ -59,7 +65,16 @@ describe('a growing reply on the phone', () => {
       mayStillGrow: working,
       activeTurnIsWorking: true
     })
-    const tree = createElement(MobileNativeChatReplyRevealsContext.Provider, { value: chat }, row)
+    const revealing = createElement(
+      MobileNativeChatReplyRevealsContext.Provider,
+      { value: chat },
+      row
+    )
+    const tree = createElement(
+      MobileNativeChatGrowthHoldContext.Provider,
+      { value: growth },
+      revealing
+    )
     act(() => {
       if (renderer) {
         renderer.update(tree)
@@ -101,6 +116,7 @@ describe('a growing reply on the phone', () => {
   afterEach(() => {
     act(() => renderer?.unmount())
     renderer = undefined
+    growth = createMobileNativeChatGrowthHold()
     vi.unstubAllGlobals()
     vi.restoreAllMocks()
     vi.useRealTimers()
@@ -128,6 +144,17 @@ describe('a growing reply on the phone', () => {
     expect(drawn()).toBe(second.length)
   })
 
+  it('waits while the page holds growth, then catches up in one step', () => {
+    growth.set(true)
+    render(first)
+    const shown = drawn()
+    advance(DELAY * 2)
+    expect(drawn()).toBe(shown)
+    act(() => growth.set(false))
+    advance(16)
+    expect(drawn()).toBe(first.length)
+  })
+
   it.each([false, true])('paces a second prose block after the first caught up: %s', (caughtUp) => {
     render(first)
     advance(caughtUp ? DELAY : DELAY / 2)

mobile/src/session/MobileNativeChatMessage.tsx

@@ -25,7 +25,7 @@ import {
   MobileNativeChatGroupedThought,
   MobileNativeChatReasoningRow
 } from './MobileNativeChatReasoningRow'
-import { mobileNativeChatRowId } from './MobileNativeChatScrollHold'
+import { mobileNativeChatRowId } from './mobile-native-chat-reading-place'
 import { nativeChatWorkRunEntries } from '../../../src/shared/native-chat-work-run'
 import { nativeChatReasoningDisclosureKey } from '../../../src/shared/native-chat-reasoning-row'
 import { MobileNativeChatTurnStatus } from './MobileNativeChatTurnStatus'