fix(ui): restore session transcript reading position

apps/desktop/e2e/transcript-scroll.spec.ts

@@ -17,7 +17,7 @@
  * under the License.
  */
 
-import { expect, test, COMPOSER_INPUT } from './fixtures';
+import { expect, test, COMPOSER_INPUT, ensureSidebarExpanded } from './fixtures';
 import type { Page } from '@playwright/test';
 
 /**
@@ -80,15 +80,15 @@ function scrollMetrics(page: Page): Promise<{
  * `toBeVisible` passes on the transparent one.
  */
 function scrollButtonOffered(page: Page): Promise<boolean> {
-  return page.evaluate((name) => {
+  return page.evaluate((names) => {
     const button = [...document.querySelectorAll('button')].find(
-      (candidate) => candidate.getAttribute('aria-label') === name
-        || candidate.textContent?.trim() === name,
+      (candidate) => names.includes(candidate.getAttribute('aria-label') ?? '')
+        || names.includes(candidate.textContent?.trim() ?? ''),
     );
-    if (!button) throw new Error(`the "${name}" affordance is missing`);
+    if (!button) throw new Error(`the "${names.join('" / "')}" affordance is missing`);
     const style = getComputedStyle(button);
     return style.pointerEvents !== 'none' && Number(style.opacity) > 0.5;
-  }, '滚动主对话到底部');
+  }, ['滚动主对话到底部', 'Scroll main conversation to bottom']);
 }
 
 function turnTop(page: Page, turnId: string): Promise<number> {
@@ -99,6 +99,48 @@ function turnTop(page: Page, turnId: string): Promise<number> {
   }, turnId);
 }
 
+function turnOffsetFromScroller(page: Page, turnId: string): Promise<number> {
+  return page.evaluate(([selector, id]) => {
+    const root = document.querySelector(selector);
+    const turn = document.querySelector(`[data-turn-id="${CSS.escape(id as string)}"]`);
+    if (!root || !turn) return Number.POSITIVE_INFINITY;
+    return Math.round(turn.getBoundingClientRect().top - root.getBoundingClientRect().top);
+  }, [SCROLLER, turnId] as const);
+}
+
+function waitForStableTurnAtScrollerStart(page: Page, turnId: string): Promise<void> {
+  return page.evaluate(([selector, id]) => new Promise<void>((resolve, reject) => {
+    let previousTop = Number.NaN;
+    let previousHeight = Number.NaN;
+    let stableFrames = 0;
+    const timeout = window.setTimeout(
+      () => reject(new Error(`Turn ${id} did not settle at the scroller start`)),
+      10_000,
+    );
+    const measure = (): void => {
+      const root = document.querySelector<HTMLElement>(selector);
+      const turn = root?.querySelector<HTMLElement>(`[data-turn-id="${CSS.escape(id)}"]`);
+      if (root && turn) {
+        const offset = turn.getBoundingClientRect().top - root.getBoundingClientRect().top;
+        stableFrames = Math.abs(offset) <= 4
+            && root.scrollTop === previousTop
+            && root.scrollHeight === previousHeight
+          ? stableFrames + 1
+          : 0;
+        previousTop = root.scrollTop;
+        previousHeight = root.scrollHeight;
+        if (stableFrames >= 6) {
+          window.clearTimeout(timeout);
+          resolve();
+          return;
+        }
+      }
+      window.requestAnimationFrame(measure);
+    };
+    measure();
+  }), [SCROLLER, turnId]);
+}
+
 /**
  * Sample the tail through the frames a growing transcript produces.
  *
@@ -206,6 +248,96 @@ test('a streaming answer keeps the viewport at the tail', async ({ window: page
   expect(await scrollButtonOffered(page)).toBe(false);
 });
 
+test('switching Sessions restores a Turn anchor while a tail Session follows background growth', async ({
+  promptRailWindow: page,
+}) => {
+  test.slow();
+  await page.setViewportSize({ width: 900, height: 700 });
+  await ensureSidebarExpanded(page);
+  const rows = page.locator('.maka-session-row');
+  const selected = rows.locator('button.astryx-side-nav-item.selected');
+  const readingSessionId = await selected.evaluate(
+    (button) => button.closest('.maka-session-row')?.getAttribute('data-session-id'),
+  );
+  if (!readingSessionId) throw new Error('the prompt-rail Session is not selected');
+  const tailSessionId = await rows.evaluateAll(
+    (items, active) => items
+      .map((row) => row.getAttribute('data-session-id'))
+      .find((sessionId) => sessionId !== active) ?? null,
+    readingSessionId,
+  );
+  if (!tailSessionId) throw new Error('the fixture has no tail-intent Session');
+  const rowButton = (sessionId: string) => page.locator(
+    `.maka-session-row[data-session-id=${JSON.stringify(sessionId)}] button`,
+  ).first();
+
+  // Move the active bounded range away from the latest window, then establish
+  // a reading position through the real scroll event path. Reopening the
+  // Session now has to use the saved sequence before the Turn can exist.
+  await page.locator('.maka-prompt-rail-tick').first().click({ force: true });
+  const readingTurnId = 'turn-prompt-rail-1';
+  await expect(page.locator(`[data-turn-id="${readingTurnId}"]`)).toHaveCount(1, {
+    timeout: 20_000,
+  });
+  await expect.poll(
+    async () => Math.abs(await turnOffsetFromScroller(page, readingTurnId)),
+    { message: 'the unloaded prompt reaches the scroller start' },
+  ).toBeLessThanOrEqual(24);
+  // The prompt rail holds its target through late content measurement. Six
+  // unchanged painted frames exceed its own quiet-frame handoff, so switching
+  // after this point cannot carry that release into the next Session. Observe
+  // transcript geometry directly: the Astryx dock can be briefly unmounted on
+  // a loaded Xvfb worker even though the reading position is already stable.
+  await waitForStableTurnAtScrollerStart(page, readingTurnId);
+  expect(await distanceToTail(page)).toBeGreaterThan(100);
+
+  await rowButton(tailSessionId).click();
+  await expect(rowButton(tailSessionId)).toHaveClass(/selected/);
+  await waitForPaintedFrames(page, 6);
+  expect(await distanceToTail(page)).toBeLessThanOrEqual(4);
+  // The fixture Session predates connection identities. Choosing any current
+  // model upgrades it onto the E2E Runtime Host before this test starts a Turn.
+  const modelSwitcher = page.locator('.maka-model-switcher-trigger');
+  await modelSwitcher.click();
+  await page.getByRole('menuitemradio', { name: 'glm-4.5', exact: true }).click();
+  await expect(modelSwitcher).toContainText('glm-4.5');
+
+  await page.evaluate((sessionId) => {
+    const state = { complete: false, unsubscribe: () => undefined };
+    state.unsubscribe = window.maka.sessions.subscribeEvents(sessionId, (event) => {
+      if (event.type !== 'complete') return;
+      state.complete = true;
+      state.unsubscribe();
+    });
+    (window as typeof window & { __makaBackgroundTailProbe?: typeof state })
+      .__makaBackgroundTailProbe = state;
+  }, tailSessionId);
+  await sendPrompt(page, LONG_PROMPT);
+  await expect(page.locator('.maka-user-message', { hasText: '第 1 行' })).toBeVisible();
+
+  // The transcript collapses before each async replacement. This round trip
+  // therefore exercises the production ordering that made a saved scrollTop
+  // become zero, not merely a pre-filled test tree.
+  await rowButton(readingSessionId).click();
+  await expect.poll(
+    async () => Math.abs(await turnOffsetFromScroller(page, readingTurnId)),
+    { timeout: 20_000, message: 'the saved reading Turn returns to the scroller start' },
+  ).toBeLessThanOrEqual(4);
+  expect(await scrollButtonOffered(page)).toBe(true);
+
+  await expect.poll(() => page.evaluate(() => (
+    window as typeof window & {
+      __makaBackgroundTailProbe?: { complete: boolean };
+    }
+  ).__makaBackgroundTailProbe?.complete === true), { timeout: 30_000 }).toBe(true);
+  await rowButton(tailSessionId).click();
+  await expect(rowButton(tailSessionId)).toHaveClass(/selected/);
+  await waitForPaintedFrames(page, 6);
+  const tail = await scrollMetrics(page);
+  expect(tail.distance, JSON.stringify(tail)).toBeLessThanOrEqual(4);
+  expect(await scrollButtonOffered(page)).toBe(false);
+});
+
 /**
  * The turn wrappers are not the transcript. It also renders the optimistic user
  * message, the no-tail live fallback and orphaned conversation items outside

apps/desktop/renderer-architecture.json

@@ -733,27 +733,18 @@
         "nonTriviaTokens": 650
       },
       "src/renderer/app-shell-session-ui-state.ts": {
-        "importDeclarations": 6,
+        "importDeclarations": 0,
         "bridgePaths": {},
         "environmentCapabilities": {},
-        "hookCalls": {
-          "useRef": 1
-        },
+        "hookCalls": {},
         "lifecycleMethods": {},
         "unresolvedDependencies": 0,
-        "actionFactories": [
-          "createAppShellSessionUiStateController"
-        ],
+        "actionFactories": [],
         "dependencyPaths": {
-          "./observable-state.js": 1,
-          "./shell-run-update-state.js": 1,
-          "@maka/core/events": 1,
-          "@maka/core/session-event-health": 1,
-          "@maka/ui": 1,
-          "react": 1
+          "./features/conversation/index.js": 1
         },
-        "importSpecifiers": 8,
-        "nonTriviaTokens": 1183
+        "importSpecifiers": 0,
+        "nonTriviaTokens": 7
       },
       "src/renderer/app-shell-stop-action.ts": {
         "importDeclarations": 4,
@@ -949,6 +940,7 @@
           "./desktop-execution-boundary-surface": 1,
           "./desktop-slash-command": 1,
           "./error-boundary": 1,
+          "./features/conversation": 1,
           "./features/goals": 1,
           "./features/module-hub": 1,
           "./features/session-collaboration": 1,
@@ -978,7 +970,6 @@
           "./settings/runtime-host-ssh-terminal-dialog.js": 1,
           "./settings/tasks-settings-page": 1,
           "./stale-sessions": 1,
-          "./task-readiness-notice": 1,
           "./use-active-execution-boundary": 1,
           "./use-app-shell-composer-quotes": 1,
           "./use-app-shell-session-ui-reads": 1,
@@ -1023,7 +1014,7 @@
           "react": 1
         },
         "importSpecifiers": 186,
-        "nonTriviaTokens": 15779
+        "nonTriviaTokens": 15725
       },
       "src/renderer/use-app-shell-composer-quotes.ts": {
         "importDeclarations": 3,
@@ -1074,7 +1065,7 @@
         "nonTriviaTokens": 581
       },
       "src/renderer/use-app-shell-session-ui-reads.ts": {
-        "importDeclarations": 4,
+        "importDeclarations": 3,
         "bridgePaths": {},
         "environmentCapabilities": {},
         "hookCalls": {
@@ -1086,11 +1077,10 @@
         "dependencyPaths": {
           "./app-shell-session-ui-state.js": 1,
           "./live-turn-snapshot.js": 1,
-          "./use-external-store-selector.js": 1,
-          "@maka/ui": 1
+          "./use-external-store-selector.js": 1
         },
-        "importSpecifiers": 10,
-        "nonTriviaTokens": 329
+        "importSpecifiers": 7,
+        "nonTriviaTokens": 322
       },
       "src/renderer/use-app-shell-session-workspace.ts": {
         "importDeclarations": 11,
@@ -4612,9 +4602,7 @@
         "unresolvedDependencies": 0,
         "actionFactories": [],
         "dependencyPaths": {
-          "@maka/core/session-send-projection": 1,
-          "@maka/core/task-submission-readiness": 1,
-          "@maka/core/ui-locale": 1
+          "./features/conversation/index.js": 1
         }
       },
       "src/renderer/theme.ts": {