fix(web): PR code tab no longer reloads when turns finish or the PR updates

[Gigioxx]

Problem

Fixes #14483. Replaces #14513, which was closed for incomplete UI verification; this adds multi-page recordings and fixes the CodeRabbit finding there. The PR panel's Code tab threw away every loaded diff page and showed "Loading pull request diff..." whenever a turn finished anywhere in the environment, or when the pull request's updatedAt moved (comments, reviews, pushes). Both background signals reused the Refresh button's token. A reader lost the diff and their scroll position mid-review even when nothing in it changed.

Change

The Code tab now takes two tokens:

• refreshToken: the Refresh button only. Same hard reset as before.
• backgroundRefreshToken: finished turns plus new updatedAt revisions. Every loaded page stays on screen. The tab re-reads them in order from the first page, and only accepts answers fetched after the refresh began, so a cached page cannot vouch for itself. An unchanged page moves the walk to the next one. A changed page replaces itself and drops the pages after it, since their cursors pointed into the old diff. Loading the next page waits until the walk catches up.

The slice reconcile step moved into reconcileDiffSlices in pullRequestDiff.logic.ts so it can be tested on its own.

This supersedes the turn-only part of #13835 and also covers the updatedAt path described in the triage.

Verification

Isolated Playwright browser against a worktree dev server with fresh .t3 state. A second tab sent a one-line prompt in an unrelated thread, so a turn completed in the same environment while the Code tab was open.

Multi-page: #14215, 476 files across 5 diff pages, all loaded, scrolled near the bottom.

	Files in Code tab	Diff scroll position	Loading state
Before (main)	476 to 100+	13732 to 0	pages dropped
After	476 throughout	13732 throughout	never shown

• Recording, before: multipage-before.mp4
• Recording, after: multipage-after.mp4

Before, after the turn	After, after the turn

A temporary console trace (removed before commit) confirmed the walk in the after run. Each of pages 1 through 5 first saw its cached answer, refreshed it, then advanced once the fresh answer matched.

Single page: #14497. Before, the diff unmounted to the loading skeleton with "0 files". After, it stayed mounted with no loading state, across repeated turns.

Before	After

Tests and checks:

• vp test run apps/web/src/components/pullRequest/pullRequestDiff.logic.test.ts: 17 passed. The new cases cover appending a page, walking forward through unchanged pages, and replacing a changed later page while dropping the pages after it.
• Web typecheck is clean. Lint shows no new warnings in the changed files.
• A Codex review of the branch found no actionable issues.

Not checked live:

• The updatedAt path. It needs a pull request that changes while open, and it uses the same token as the turn path.
• A later page whose content actually changes.

Unrelated, seen on main too: the first diff read sometimes fails with a 503 when an earlier request is aborted (499) while a shared fetch is in flight. Reloading recovers.

Made with Claude Opus 5.5 in Claude Code (via T3 Code).

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📥 Commits

Reviewing files that changed from the base of the PR and between c57a04b and 699de7d.

📒 Files selected for processing (4)

• apps/web/src/components/pullRequest/PullRequestCodeTab.tsx
• apps/web/src/components/pullRequest/PullRequestDetailPanel.tsx
• apps/web/src/components/pullRequest/pullRequestDiff.logic.test.ts
• apps/web/src/components/pullRequest/pullRequestDiff.logic.ts

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[macroscopeapp]

Approvability

Verdict: Approved at 699de7d

Macroscope's review found this PR approvable — This is a localized, tested fix to preserve loaded pull-request diff pages during background updates while retaining the existing manual refresh reset. Its runtime impact is confined to the PR Code tab, with no schema, infrastructure, security, billing, or configuration-sensitive changes.

Notes:

• Diff unchanged. Approvability was decided on eligibility alone.

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Dismissing prior approval to re-evaluate 699de7d