fix(web): keep older trajectory history loadable

apps/web/tests/snapshots/trajectory-virtualization/load-more.expected.md

@@ -0,0 +1,3 @@
+- row "Load earlier history":
+  - cell "Load earlier history":
+    - button "Load earlier history"

apps/web/tests/trajectory-virtualization.e2e.ts

@@ -3,6 +3,7 @@
 // DOM mounting stays bounded, and every scroll range remains reachable.
 import { mkdtemp, rm, writeFile } from 'node:fs/promises'
 import { tmpdir } from 'node:os'
+import { fileURLToPath } from 'node:url'
 import { join } from 'node:path'
 import type { Browser, Page } from 'playwright'
 import { chromium } from 'playwright'
@@ -11,6 +12,8 @@ import type { StreamChunk } from '@deepseek-ai/dsh-llm'
 import type { ReplayEntry } from '@deepseek-ai/dsh-llm-replay'
 import { createChatScrollFixture } from './chat-scroll-fixture.ts'
 import {
+  captureStableAria,
+  compareOrRefreshGolden,
   launchWebScaffold,
   seedSession,
   watchConsole,
@@ -20,6 +23,10 @@ import {
 import { newEnglishPage, saveFailureShot } from './support.ts'
 
 const MODE = webSnapshotMode()
+const LOAD_MORE_EXPECTED = fileURLToPath(new URL(
+  './snapshots/trajectory-virtualization/load-more.expected.md',
+  import.meta.url,
+))
 const SESSION_ID = 'trajectory-virtualization-e2e'
 const FIXTURE = createChatScrollFixture({
   markerPrefix: 'TRAJECTORY_VIRTUAL',
@@ -230,8 +237,26 @@ describe('web e2e: Trajectory virtualization over tail-paged history', () => {
       expect(await page.getByText('Initial System Prompt', { exact: true }).count()).toBe(0)
       expect(await mountedRows(page)).toBeLessThanOrEqual(MAX_MOUNTED_ROWS)
 
-      await scrollToRatio(page, 0)
+      const loadMore = page.locator('[data-history-load] button')
+      await loadMore.waitFor({ timeout: 15_000 })
+      expect(await loadMore.textContent()).toBe('Load earlier history')
+      const loadMoreSnapshot = await captureStableAria(
+        page,
+        '[data-history-load]',
+        scaffold.workspaceCwd,
+      )
+      await compareOrRefreshGolden(LOAD_MORE_EXPECTED, loadMoreSnapshot, MODE)
+      await loadMore.evaluate((button: HTMLButtonElement) => { button.click() })
       await expect.poll(() => held, { timeout: 15_000 }).toBe(true)
+      await expect.poll(async () => ({
+        disabled: await loadMore.isDisabled(),
+        label: await loadMore.textContent(),
+      }), { timeout: 15_000 }).toEqual({
+        disabled: true,
+        label: 'Loading earlier history…',
+      })
+
+      await scrollToRatio(page, 0)
       const anchor = await firstVisibleRow(page)
       const selectedRow = page.locator(
         `[data-trajectory-scroll] tr[data-trajectory-row-key=${JSON.stringify(anchor.key)}]`,