#22901 Fix the six most common macOS hang patterns from Sentry

This PR fixes the six most common macOS app-hang patterns in Sentry over the last 14 days (0.65.x). Each pattern has its own issue, its own commits, and a regression test.

Pattern	Issue	Fix
P1 SwiftUI update storm (largest; about 27% of 0.65.2 devices)	https://github.com/manaflow-ai/cmux/issues/22703	The Bonsplit split root kept one withObservationTracking registration per SwiftUI update. After 25 updates, one change re-rendered the tree 26 times. It now keeps one live registration (https://github.com/manaflow-ai/bonsplit/pull/291, carried by https://github.com/manaflow-ai/bonsplit/pull/292).
P2 session checkpoint and autosave on main	https://github.com/manaflow-ai/cmux/issues/22704	Per-candidate settings paths, UserDefaults reads, process-table lookups and file removal are hoisted or moved off the main thread.
P3 terminal portal re-attach	https://github.com/manaflow-ai/cmux/issues/22705	Re-attaching an unchanged surface skips geometry, background, blur, accessibility and active-space work.
P4 sidebar rows	https://github.com/manaflow-ai/cmux/issues/22706	The row height cache ignores chrome-only fields, NSTextView is used only for Markdown lines, heights are noted selectively, and WorkstreamItem lookups use a request-id index.
P5 welcome carousel	https://github.com/manaflow-ai/cmux/issues/22708	https://github.com/manaflow-ai/cmux/pull/25585 already landed the cached playhead on main. What remains here takes the pager's duration from the loaded asset instead of the player.
P6 small blocking calls on main	https://github.com/manaflow-ai/cmux/issues/22709	Fixes in activation policy, hotkey keyboard layout, PaneMemoryGuardrail defaults, socket marker write, remote feature flags, and the browser history merge (was O(n²)). Skipped: 3WW9 (scrubber; moving capture off main is a policy decision, https://github.com/manaflow-ai/cmux/issues/12790) and 20CS (the time is AppKit's own icon redraw; no safe fix without measuring).

Related: https://github.com/manaflow-ai/cmux/issues/21260, https://github.com/manaflow-ai/cmux/pull/20250, https://github.com/manaflow-ai/cmux/issues/21352, https://github.com/manaflow-ai/cmux/issues/12790. This does not cover https://github.com/manaflow-ai/cmux/pull/21451, which landed separately and does not fix P1.

Merge order:

1. https://github.com/manaflow-ai/bonsplit/pull/292, with a real merge commit, so 2803bcc stays reachable from Bonsplit main. It contains https://github.com/manaflow-ai/bonsplit/pull/291 and the pointer main pins today (58b73fd), which is not on Bonsplit main either.
2. This PR, after checking that vendor/bonsplit (2803bcc) is reachable from Bonsplit main. If https://github.com/manaflow-ai/bonsplit/pull/292 is squashed, re-pin to the landed SHA first.

Complements https://github.com/manaflow-ai/cmux/pull/22748, which also targets https://github.com/manaflow-ai/cmux/issues/22703. That PR stops the idle agent-index reload bursts that re-render every pane. This PR's Bonsplit change stops each of those renders from multiplying through leaked observation registrations. No files overlap.

Testing

Current head 5025858, P1 against today's code:

• Red: Bonsplit run 41941235173 at 35f9009, which is cmux main's pointer 58b73fd plus only the test. SplitTreeObservationTests failed: 26 and 53 tree renders where 1 and 2 are expected.
• Green: Bonsplit run 40420806256 at 2803bcc, the pointer on this PR. SplitTreeObservationTests passed.
• Fleet build job 28d32972c71b59dec9a5a246 at 5025858 (tag 22703-swiftui-graph-update-hang): built and installed, and the app's CMUXCommit is 5025858db.
• PR CI run 41974550264 at 5025858: compile admission, swift-package-tests and all CLI regression shards passed, and so did five of the seven app-host shards. Shards 1/7 and 6/7 are red only on suites main also fails: Cloud Machines header count (fix: https://github.com/manaflow-ai/cmux/pull/35974) and RemoteTmuxMirrorCloseDetachTests. Both are in main run 41915901489, and this PR changes neither.
• Focused run 41952355399 at 33902b3, one test-only change set before this head: 54 tests in 3 suites passed, covering WorkspaceContentViewVisibilityTests, WorkspaceProcessWideObserverRetirementTests and CloudWelcomeTests.

The receipts below are from older heads, kept as history.

P1, Bonsplit CI, SplitTreeObservationTests:

• Red at 83a0903: 26 and 53 tree renders where 1 and 2 are expected. https://github.com/manaflow-ai/bonsplit/actions/runs/39540663317
• Green at aed1a61: https://github.com/manaflow-ai/bonsplit/actions/runs/39540846201

P2 review fixes, cmuxTests/SessionScrollbackCheckpointCoordinatorTests (15 tests):

• With the fix, at 3d0a81e: passed. https://github.com/manaflow-ai/cmux/actions/runs/39542033857
• Red at 53fe696: failed as expected. Exactly the two new tests failed (interruptedCheckpointStillRemovesThePlannedStaleFile, seedBeforeTheStartupListingIsStillTracked). https://github.com/manaflow-ai/cmux/actions/runs/39542031403

P2–P6 focused suites at 3d0a81e, on owned Macs:

• P2: passed. 37 tests in 5 suites (https://github.com/manaflow-ai/cmux/actions/runs/39542321675) and 2 tests in 2 suites (https://github.com/manaflow-ai/cmux/actions/runs/39542347356).
• P5: passed, 28 tests. https://github.com/manaflow-ai/cmux/actions/runs/39542232197
• P6: passed, 74 tests in 5 suites. https://github.com/manaflow-ai/cmux/actions/runs/39552556729
• P3: passed at 0d6daeb, after the test-setup fix that waits for the runtime surface. https://github.com/manaflow-ai/cmux/actions/runs/39567263789
• P4, three earlier failures, all now fixed:
  • First run, at 3d0a81e: https://github.com/manaflow-ai/cmux/actions/runs/39542257224. The existing test metadataRowReconfigurationClearsMutuallyExclusiveState caught a real regression, the hidden Markdown view keeping stale text and links; fixed in 9f2940a. The new test listWideRepaintNotesOnlyRowsWhoseHeightChanged first applied at clip width 0; its setup was fixed in 0d6daeb.
  • Second run, at 0d6daeb: https://github.com/manaflow-ai/cmux/actions/runs/39567284707. reapplyingUnchangedMarkdownMetadataKeepsTextStorage asserted the stale storage was kept, which contradicts the existing contract. It now expects the view to clear, at 6ce3b1c. Repeated applies of the same Markdown entry are still edit-free.
• P4 at 6ce3b1c: passed, 109 tests in 3 suites. https://github.com/manaflow-ai/cmux/actions/runs/39581316781

Observed / Not run

• Observed:
  • Bonsplit red and green runs.
  • P2 red and green runs.
  • P2–P6 focused app-host suites, all green on the head commits above.
  • cmux-ci package steps.
  • Fleet build of 3d0a81e (compiled, signed, packaged).
  • Correctness reviews (comments below).
  • Base-red checks on main for AgentLaunchEnvironmentPolicy and SettingsRowAnchorResolution (comment below).
• Not run:
  • P2-P6 focused suites were not rerun individually on 5025858; PR CI on the head covers them.
  • Field proof: no release contains this fix, so Sentry can't show its effect yet.
  • Live-app dogfood of the render storm; the regression test hosts real SwiftUI in a window instead.
  • Per-pattern red runs for P3-P6; those regression tests were committed alongside each fix.
  • P5 changes a carousel's timing source; no visual change is intended.

No visible UI change is intended.

Changelog

Fixed: fewer app hangs from split-view re-render storms, session checkpoints, terminal re-attach, sidebar rows, the Cloud welcome carousel and blocking main-thread calls.

🤖 Generated with Claude Code

Summary by CodeRabbit

• Bug Fixes

  • Improved reliability of session restoration, scrollback checkpoints, agent status tracking, and terminal visibility.
  • Preserved existing sidebar row heights during presentation-only updates, while still resizing rows when their content changes.
  • Prevented unnecessary rewrites of unchanged socket markers and redundant preference updates.
  • Improved browser history merging and shortcut key resolution.

• Performance

  • Reduced unnecessary work when updating terminal surfaces, sidebar content, and session snapshots.
  • Made request identity lookups and agent liveness checks more efficient.