Fix infinite UA-policy restart loop on Google Sheets destinations

[lintanghui]

On some macOS versions WebKit reports the native identity as "" rather than nil after a restart clears customUserAgent. The policy check customUserAgent != resolvedUserAgent then never converges for destinations resolved to webKitDefault (like docs.google.com), so every policy pass cancels the navigation and reloads — the Sheets pane hangs and the main thread spins at ~100% CPU.

• Treat an empty reported identity the same as nil when comparing against the resolved policy, so webKitDefault destinations no longer register a mismatch.
• As a backstop, remember the last main-frame URL we restarted for and allow at most one restart in a row per destination; if the replacement load still reports a mismatch, let it proceed instead of looping.
• Reworked restartNavigationForBrowserUserAgentPolicyIfNeeded to take the request and main-frame flag directly so the loop guard is testable without constructing a WKNavigationAction.

Added tests covering the empty-identity case and the restart-at-most-once behavior.

Fixes #9462

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Summary by cubic

Fixes an infinite user‑agent policy restart loop on Google Sheets (docs.google.com) that froze the pane and pegged CPU on some macOS versions. Treats empty identities correctly and limits restarts so navigation proceeds. Fixes #9462.

• Bug Fixes

  • Treat empty customUserAgent as nil when comparing against webKitDefault.
  • Allow at most one UA-policy restart per main-frame destination; if the replacement still mismatches, proceed.
  • Added tests for the empty-identity case and the single-restart behavior.

• Refactors

  • Updated restartNavigationForBrowserUserAgentPolicyIfNeeded to take request and targetFrameIsMainFrame directly for easier testing and clearer logic.
  • Track the last restarted URL via an associated object to implement the loop guard.

^{Written for commit 1f93491. Summary will update on new commits.}

[coderabbitai]

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ℹ️ Recent review info

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Run ID: 6a17998e-5fd0-47c6-be03-b6c7b163b787

📥 Commits

Reviewing files that changed from the base of the PR and between a2d28ba and 1f93491.

📒 Files selected for processing (4)

• Sources/Panels/BrowserNavigationDelegate.swift
• Sources/Panels/BrowserPopupWindowController.swift
• Sources/Panels/WKWebView+BrowserUserAgentPolicy.swift
• cmuxTests/BrowserUserAgentPolicyWebKitTests.swift

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📝 Walkthrough

Walkthrough

The browser user-agent policy now treats an empty custom user agent as the WebKit default and prevents repeated restarts for the same navigation destination. Navigation delegates pass requests and main-frame information directly to the policy. Tests cover Sheets navigation and later restart eligibility.

Changes

Browser user-agent policy

Layer / File(s)	Summary
User-agent normalization and restart state Sources/Panels/WKWebView+BrowserUserAgentPolicy.swift	Empty customUserAgent values resolve as the WebKit default. The web view stores the destination of the latest policy restart.
Guarded navigation restart flow Sources/Panels/WKWebView+BrowserUserAgentPolicy.swift, Sources/Panels/BrowserNavigationDelegate.swift, Sources/Panels/BrowserPopupWindowController.swift, cmuxTests/BrowserUserAgentPolicyWebKitTests.swift	Restart handling accepts a URLRequest and main-frame flag, skips repeated restarts for the same URL, and allows a later navigation to restart again. Callers and tests use the updated flow.

Estimated code review effort: 2 (Simple) | ~10 minutes

Possibly related PRs

• manaflow-ai/cmux#9054: Introduced the WebKit user-agent restart logic extended by this change.
• manaflow-ai/cmux#8697: Changed embedded WebKit user-agent configuration used by the same browser policy.
• manaflow-ai/cmux#8939: Also modifies navigation handling in BrowserPopupWindowController.

Suggested reviewers: azooz2003-bit

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Important

Pre-merge checks failed

Please resolve all errors before merging. Addressing warnings is optional.

❌ Failed checks (2 errors, 1 warning)

Check name	Status	Explanation	Resolution
Cmux Architecture Rethink	❌ Error	The new associated-object URL cache is a mutable side channel for navigation state already owned by the navigation delegates, so the mismatch remains representable and only the Sheets symptom is ca...	Move the restart-attempt invariant into one shared navigation coordinator or WebView-owned model, pass it through both delegates, and clear it on explicit navigation completion or cancellation.
Cmux No Ambient Global State	❌ Error	The diff adds private var cmuxBrowserUserAgentPolicyRestartedURLKey: UInt8 = 0 at file scope (line 5), a new top-level mutable variable prohibited by the rule.	Move restart tracking and its association-key mechanism into a scoped, constructable BrowserUserAgentPolicyState owned by the browser delegate/web view seam and inject that state; remove the file-scope mutable variable.
Description check	⚠️ Warning	The description explains the cause and fix but omits the required demo video, review trigger, checklist, and manual testing details.	Add the missing template sections, document manual verification, include a demo video or explain its omission, and complete the checklist.

✅ Passed checks (22 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly identifies the primary change: fixing the infinite user-agent policy restart loop affecting Google Sheets.
Linked Issues check	✅ Passed	The changes directly address issue #9462 by preventing repeated Sheets navigation restarts and adding tests for the reported failure modes.
Out of Scope Changes check	✅ Passed	All changed files and tests support the user-agent policy restart-loop fix described in issue #9462.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Cmux Swift Actor Isolation	✅ Passed	Production changes remain in @MainActor WKWebView methods and @MainActor navigation delegates; no new models, service protocols, Sendable reference types, or background UI-store access were introdu...
Cmux Swift Blocking Runtime	✅ Passed	The production diff adds no semaphore, wait, sleep, delayed dispatch, polling, main-queue sync, or manual lock; it only adds synchronous URL state tracking. Tests are deterministic.
Cmux Browser Automation Off-Main	✅ Passed	The PR changes only main-actor WebKit user-agent navigation and tests; it adds no browser socket command, wait, worker route, or change to the policy/router target files.
Cmux Expensive Synchronous Load	✅ Passed	The production diff only changes WebKit user-agent policy and URL bookkeeping; it adds no agent-history, transcript, JSON/JSONL, directory, or synchronous file loads on interactive main-actor paths.
Cmux Cache Substitution Correctness	✅ Passed	The diff does not replace a persistence/history/undo/snapshot read. The associated URL is transient WKWebView loop-guard state; policy resolution still reads each request and resets on a different...
Cmux No Hacky Sleeps	✅ Passed	The diff changes only Swift production code and Swift tests; the rule explicitly scopes TypeScript, JavaScript, shell, and non-Swift runtime scripts.
Cmux Algorithmic Complexity	✅ Passed	The production diff adds only constant-time URL equality, optional-string normalization, and associated-object access; it introduces no scalable collection scan, nested loop, sort, filter, or batch...
Cmux Swift Concurrency	✅ Passed	The diff adds no Dispatch, Combine, fire-and-forget Task, or new completion-handler async pattern; it only refactors existing synchronous closures at the WebKit decision-handler boundary.
Cmux Swift @Concurrent	✅ Passed	The diff adds no async, nonisolated, or @concurrent work. Changed policy methods are synchronous @MainActor functions, so the Swift concurrency rule is not violated.
Cmux Swift Package Boundaries	✅ Passed	The diff keeps pure BrowserUserAgentPolicy in CmuxBrowser; changes are WKWebView/Objective-C state and navigation-delegate glue in Sources/Panels, which the rule allows.
Cmux Swiftpm Lockfiles	✅ Passed	The PR changes only four Swift source/test files; no Package.swift, Package.resolved, .gitignore, workflow, Xcode project, or dependency files changed.
Cmux Swift Logging	✅ Passed	The patch adds no print, debugPrint, dump, NSLog, file/stdout logging, Logger, or sensitive-data diagnostics; existing NSLog calls are unchanged.
Cmux User-Facing Error Privacy	✅ Passed	The production diff changes navigation policy logic only; added vendor/issue references are developer comments, and the new user-agent cases are tests. No user-facing error or recovery text was add...
Cmux Full Internationalization	✅ Passed	The diff adds UA policy logic, comments, and tests only; it adds no user-facing production text and changes no localization catalogs, locale files, or web messages.
Cmux Swiftui State Layout	✅ Passed	The diff contains no SwiftUI views or state/layout patterns; it changes AppKit/WebKit navigation policy code and tests only.
Cmux Swift Auxiliary Window Close Shortcuts	✅ Passed	The PR changes UA navigation logic and tests only; it adds no window code. Existing browser popup uses cmux.browser-popup, is registered, and the auxiliary-window lint passes.
Cmux Source Artifacts	✅ Passed	All four changed paths are intentional Swift source or test files; the diff adds no logs, caches, scratch directories, build output, screenshots, or other local artifacts.
Cmux No Test Or Debug Seam In Production Source	✅ Passed	Production additions contain no test/debug guard or test-shaped member; the refactored restart method has callers in both production delegates, and restart state remains private.

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