Add opt-in split sidebar workspace snapshots

[Eridanus117]

Summary

This PR adds a default-off sidebar performance mode that splits workspace row snapshots into two invalidation domains:

• summary snapshot: title-adjacent visible state, metadata, logs, progress, remote state, ports
• structured details snapshot: branch/directory and pull-request rows

The legacy sidebar observation path is left intact and remains the default. The new path only activates when sidebar.performance.optimizedWorkspaceSnapshots.enabled is true and mode is splitView.

Mini-RFC

Problem

After #6460 removed the transcript scan from normal sidebar rendering, expensive residual work can still happen when frequent summary/title/log/progress updates cause each row to rebuild branch/directory/PR snapshots. Those snapshots call through ordering and panel-detail helpers such as sidebarOrderedPanelIds, sidebarBranchDirectoryEntriesInDisplayOrder, and PR display building. In LazyVStackLayout, that can still amplify into visible sidebar CPU even when the current change does not affect branch/directory/PR rows.

Approach

Keep the current default behavior unchanged, but add an opt-in split mode:

{
  "sidebar": {
    "performance": {
      "optimizedWorkspaceSnapshots": {
        "enabled": false,
        "mode": "splitView",
        "summaryDebounceMilliseconds": 40,
        "detailDebounceMilliseconds": 40,
        "diagnostics": false,
        "logInvalidationSource": false
      }
    }
  }
}

In split mode:

• TabItemView subscribes to a lighter summary publisher for the parent row snapshot.
• Branch/directory and PR rows render through a nested structured-details section with its own detail publisher and debounce.
• The old combined sidebarObservationPublisher and legacy makeWorkspaceSnapshot path remain unchanged for users who do not enable the new setting.
• Finder context-menu directory resolution is lazy in split mode, so normal summary updates do not force that detail lookup.

Rollout

1. Ship default-off.
2. Enable in nightly / affected profiles for validation.
3. Use the diagnostics flags only when investigating invalidation source behavior.
4. Consider changing the default in a later PR only after profiling confirms the split path is stable.

Tests / Verification

Passed:

• git diff --check
• xcrun swiftc -parse Sources/ContentView.swift Sources/WorkspaceSidebarObservation.swift Sources/Workspace.swift Sources/KeyboardShortcutSettingsFileStore.swift Sources/CmuxSettingsJSONPathSupport.swift Sources/KeyboardShortcutSettingsFileStore+Template.swift cmuxTests/WorkspaceSidebarObservationTests.swift cmuxTests/KeyboardShortcutSettingsFileStoreStartupTests.swift

Blocked before exercising this PR's tests:

• swift test --package-path Packages/macOS/CmuxSettings --filter UserDefaultsSettingsClientTests -Xswiftc -strict-concurrency=minimal
  • blocked by existing Swift 6 Sendable errors in Packages/macOS/CmuxFoundation/Sources/CmuxFoundation/Process/CommandRunner.swift and Packages/macOS/CmuxFoundation/Sources/CmuxFoundation/FileWatch/FileWatcher.swift
• xcodebuild -project cmux.xcodeproj -scheme cmux -destination 'platform=macOS' -skipPackageUpdates -only-testing:cmuxTests/WorkspaceSidebarObservationTests test
  • blocked before compile while SwiftPM attempted to clone https://github.com/PostHog/posthog-ios.git into the package repository cache

Related

Refs #6546
Related to #6460 and #2527

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Summary by cubic

Adds an opt-in split sidebar snapshot mode that isolates summary updates from branch/directory/PR details to cut row rebuilds and CPU during frequent updates. Legacy stays default; enable via sidebar.performance.optimizedWorkspaceSnapshots (supports #6546).

• New Features

  • New settings under sidebar.performance.optimizedWorkspaceSnapshots: enabled, mode ("splitView"), summaryDebounceMilliseconds, detailDebounceMilliseconds, diagnostics, logInvalidationSource.
  • Summary uses a lightweight publisher; branch/directory/PR rows render in a nested details section with its own publisher and debounce.
  • Finder directory resolution is lazy in split mode, so summary updates don’t trigger it.
  • Added sidebarSummaryObservationPublisher and sidebarStructuredDetailObservationPublisher; updated settings template and JSON parsing (mode normalization, debounce clamping); tests cover publisher separation and settings import.

• Bug Fixes

  • Fixed Swift 6 concurrency imports by adding @preconcurrency import Dispatch in FileWatcher and CommandRunner.

^{Written for commit f1aafd7. Summary will update on new commits.}

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[Eridanus117]

This draft is being superseded by #6564 for the direct title-driven workspace-list fanout path. The useful idea to keep around is splitting sidebar summary/detail observation if row-level rendering churn remains after the core title/mobile fanout fixes land.