Fix sidebar tab selection highlight timing

[austinywang]

Summary

Closes #6626

Fixes the vertical-tab visual regression where selecting a workspace paints in two visible phases: first the translucent multi-selected background, then the solid active selected background a moment later.

Regression provenance

The last known-good tag is v0.64.16 (e647c3049c03786299d81f3c287571373d635269). Diff/blame against HEAD points to 5ea81da1a5 / PR #6460 (Fix title-churn beachball in transcript adoption and sidebar rows) as the selection-highlight regression source.

That PR intentionally moved active workspace selection out of the VerticalTabsSidebar parent and into row-local TabItemView state so a selected-workspace change only invalidates the previous and new rows instead of every row under the ForEach. The problem was the row-local state pipeline added .receive(on: RunLoop.main), while the click path still updates selectedTabIds synchronously. On a normal click, the row first rendered as merely multi-selected (opacity: 0.25), then the scheduled active-state update arrived and repainted the solid selected fill (opacity: 1).

Fix

TabManager is @MainActor and sends selectedTabIdPublisher from its selected-workspace will-change hook, so the row-local active-state update is already main-thread delivered. Removing the extra run-loop scheduler hop keeps #6460's scoped two-row invalidation, but lets the active-state change settle in the same interaction transaction as the sidebar selection set. The selected row now paints the solid selection highlight immediately in one phase.

Verification

• git diff --check
• Inspected v0.64.16..HEAD diffs/blame for ContentView.swift, SidebarAppearanceSupport.swift, and TabManager.swift.
• No regression test added: this is a SwiftUI frame-ordering/visual timing bug, and a meaningful behavioral test would require an app/UI runtime build. A source-shape test would not prove the user-visible flash is fixed.
• No local/cloud app build or launch was run per the issue handoff instructions; the sanctioned dev build is intentionally deferred until CI is green and the user explicitly asks for it.

Localization

No user-facing strings were added or changed.

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Summary by cubic

Fixes the sidebar vertical-tab highlight lag that briefly shows a translucent highlight before the solid active state when selecting a workspace (fixes #6626). Removes the extra .receive(on: RunLoop.main) in TabItemView so TabManager’s main-thread selectedTabIdPublisher updates the active state in the same interaction, preventing the two-phase paint.

^{Written for commit e22a1ac. Summary will update on new commits.}

Summary by CodeRabbit

• Bug Fixes
  • Improved tab switching responsiveness by optimizing how selection state updates are processed, reducing unnecessary duplicate operations.

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Project	Deployment	Actions	Updated (UTC)
cmux	Ready	Preview, Comment	Jun 23, 2026 12:46am
cmux-staging	Building	Preview, Comment	Jun 23, 2026 12:46am

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: ASSERTIVE

Plan: Pro

Run ID: 07d35142-04f6-4e03-98df-79119bb771e7

📥 Commits

Reviewing files that changed from the base of the PR and between c19767a and e22a1ac.

📒 Files selected for processing (1)

• Sources/ContentView.swift

💤 Files with no reviewable changes (1)

• Sources/ContentView.swift

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📝 Walkthrough

Walkthrough

In TabItemView, the .onReceive pipeline for tab selection state is updated to map the incoming selectedTabId to a Bool ($0 == tab.id), apply .removeDuplicates(), and remove the .receive(on: RunLoop.main) step before calling updateObservedActiveState(isSelected).

Changes

Tab Selection Publisher Deduplication

Layer / File(s)	Summary
TabItemView selection pipeline Sources/ContentView.swift	Adds .map { $0 == tab.id } and .removeDuplicates() to the onReceive pipeline, and removes .receive(on: RunLoop.main). This prevents updateObservedActiveState from being called redundantly with the same isSelected value, eliminating the two-phase highlight.

Estimated code review effort

🎯 1 (Trivial) | ⏱️ ~3 minutes

Poem

🐇 A tab once blushed twice — first shy, then bold,
A duplicate signal, a stutter of old.
One removeDuplicates() hop set things right,
The highlight now snaps with a single crisp bite.
No lag, no translucent ghost haunting the view —
Just one solid blue, clean, immediate, true! ✨

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Important

Pre-merge checks failed

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

❌ Failed checks (1 error)

Check name	Status	Explanation	Resolution
Cmux Source Artifacts	❌ Error	PR adds 28 files from hidden scratch directories (.claude/ and .greptile/) that violate source-control-artifacts.md rule, including .claude/scheduled_tasks.lock (session lock with PID/timestamp) an...	Remove all files from .claude/ and .greptile/ directories, and add these patterns to .gitignore to prevent future commits of local development tool artifacts.

✅ Passed checks (22 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title accurately summarizes the main change: removing the extra scheduler hop in the tab selection state pipeline to fix the two-phase highlight rendering bug.
Description check	✅ Passed	The description provides comprehensive context including the root cause analysis, regression source, fix details, and verification steps. However, it does not include a demo video, which is specified in the template.
Linked Issues check	✅ Passed	The PR directly addresses issue #6626 by removing the .receive(on: RunLoop.main) scheduler hop that caused the two-phase highlight rendering, restoring single-phase selection behavior as expected in version 0.64.16.
Out of Scope Changes check	✅ Passed	The change is narrowly scoped to fix the identified regression in TabItemView's subscription handling. No unrelated modifications or out-of-scope changes are present.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Cmux Swift Actor Isolation	✅ Passed	Change removes redundant .receive(on: RunLoop.main) from TabItemView's subscription to TabManager's @MainActor selectedTabIdPublisher; no new actor isolation issues introduced, only redundant synch...
Cmux Swift Blocking Runtime	✅ Passed	PR removes .receive(on: RunLoop.main) scheduler hop, which decreases blocking/timing synchronization rather than introducing it. No blocking runtime primitives added.
Cmux Expensive Synchronous Load	✅ Passed	PR removes .receive(on: RunLoop.main) from TabItemView.selectedTabIdPublisher subscription—a timing fix that improves performance, not adds expensive loads. No agent session loading, JSON parsing,...
Cmux Cache Substitution Correctness	✅ Passed	PR removes .receive(on: RunLoop.main) scheduler hop from TabItemView's UI state pipeline, not a cache substitution in persistence/undo/snapshot/history path—no rule violation.
Cmux No Hacky Sleeps	✅ Passed	PR contains only Swift code changes (main fix in ContentView.swift) and prototype development scaffolding. The rule covers TypeScript/JavaScript/shell only; Swift timing is covered separately. Adde...
Cmux Algorithmic Complexity	✅ Passed	PR removes one line (.receive(on: RunLoop.main)) from TabItemView's selectedTabIdPublisher subscription, reducing scheduler overhead. No algorithmic complexity issues; change improves efficiency...
Cmux Swift Concurrency	✅ Passed	The PR removes a .receive(on: RunLoop.main) scheduler hop from TabItemView's selectedTabIdPublisher subscription, reducing legacy async patterns. No new Dispatch queues, Combine app state, comple...
Cmux Swift @Concurrent	✅ Passed	PR removes .receive(on: RunLoop.main) from a main-actor-isolated publisher subscription. No async/concurrent work involved; closure is synchronous UI-state update only.
Cmux Swift File And Package Boundaries	✅ Passed	PR removes 1 line from existing ContentView.swift (16092 vs 16117 budgeted), an oversized file already tracked. Change is a focused UI timing bug fix (removing scheduler hop), matching allowed case...
Cmux Swiftpm Lockfiles	✅ Passed	PR complies with swiftpm-package-resolved.md: cmux-owned .gitignore files don't ignore Package.resolved, root Xcode lockfile included, vendored third-party preserves upstream policy, all non-vendor...
Cmux Swift Logging	✅ Passed	No logging violations found: zero print/debugPrint/dump/NSLog calls in Sources/ContentView.swift; all cmuxDebugLog calls are guarded by #if DEBUG per swift-logging.md rules.
Cmux User-Facing Error Privacy	✅ Passed	The PR change removes a scheduler hop (.receive(on:)) from TabItemView's selectedTabIdPublisher subscription for rendering optimization. No user-facing error messages, alerts, or privacy-sensitive...
Cmux Full Internationalization	✅ Passed	PR fixes a visual regression by removing .receive(on: RunLoop.main) from TabItemView's subscription to tabManager.selectedTabIdPublisher and adding .removeDuplicates(). This is a pure technic...
Cmux Swiftui State Layout	✅ Passed	PR removes .receive(on: RunLoop.main) from existing tabManager.selectedTabIdPublisher subscription in TabItemView—no new ObservableObject/@Published/@StateObject state introduced, only incident...
Cmux Architecture Rethink	✅ Passed	PR removes unnecessary .receive(on: RunLoop.main) scheduler hop from TabItemView's selectedTabIdPublisher subscription. This is a small correctness fix with clear ownership (TabManager @MainActor...
Cmux Swift Auxiliary Window Close Shortcuts	✅ Passed	PR only modifies TabItemView (a SwiftUI View for sidebar tab rows), not any standalone cmux-owned windows (NSWindow, NSPanel, NSWindowController, Window, or WindowGroup). Changes only remove a sche...
Cmux No Test Or Debug Seam In Production Source	✅ Passed	PR removes one line of code (.receive(on: RunLoop.main)) from Sources/ContentView.swift with no added lines. No #if DEBUG blocks, test/debug seams, or visibility widening for test access introduced.

✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch issue-6626-tab-selection-double-highlight

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[greptile-apps]

Greptile Summary

Removes the .receive(on: RunLoop.main) scheduler hop from the selectedTabIdPublisher Combine pipeline inside TabItemView, eliminating the two-phase repaint that caused a visible flash when selecting a sidebar workspace tab.

• TabManager is @MainActor and selectedTabIdPublisher is a CurrentValueSubject, so the publisher already delivers synchronously on the main thread; the extra run-loop re-dispatch was causing observedIsActive to settle one cycle late, painting the translucent multi-selected background before the solid active fill.
• The two other .receive(on: RunLoop.main) hops in the same view (sidebarImmediateObservationPublisher and sidebarObservationPublisher) are left in place; those publishers may fire from non-main contexts and sidebarObservationPublisher additionally feeds a .debounce scheduled on RunLoop.main.

Confidence Score: 5/5

Safe to merge — the one-line removal correctly eliminates an unnecessary scheduler hop from a publisher that already delivers on the main thread.

The change is a single-line deletion of .receive(on: RunLoop.main) from a Combine pipeline whose source (CurrentValueSubject on @MainActor TabManager) always fires synchronously on the main thread. Removing the hop is provably safe and closes the two-phase render race without touching any other state path. The two remaining .receive(on: RunLoop.main) calls in the same view are intentionally kept for publishers that can originate off-main.

No files require special attention.

Important Files Changed

Filename	Overview
Sources/ContentView.swift	Removes .receive(on: RunLoop.main) from the selectedTabIdPublisher pipeline; correct and minimal — TabManager is @mainactor so the publisher fires synchronously on main with no re-dispatch needed.

Sequence Diagram

%%{init: {'theme': 'neutral'}}%%
sequenceDiagram
    participant User
    participant TabManager as TabManager (@MainActor)
    participant Publisher as selectedTabIdPublisher (CurrentValueSubject)
    participant TabItemView

    Note over User,TabItemView: Before fix — two render phases
    User->>TabManager: click workspace
    TabManager->>Publisher: send(newId) [synchronous, on main]
    Publisher-->>TabItemView: .receive(on: RunLoop.main) schedules next cycle
    TabItemView->>TabItemView: render phase 1: multi-selected (opacity 0.25)
    Note over TabItemView: next RunLoop cycle
    Publisher-->>TabItemView: "callback fires, observedIsActive = true"
    TabItemView->>TabItemView: render phase 2: solid active fill (opacity 1.0)

    Note over User,TabItemView: After fix — single render phase
    User->>TabManager: click workspace
    TabManager->>Publisher: send(newId) [synchronous, on main]
    Publisher-->>TabItemView: onReceive fires in same transaction
    TabItemView->>TabItemView: "observedIsActive = true, single solid render"

Loading

%%{init: {'theme': 'base', 'themeVariables': {"darkMode": true, "background": "#0d1117", "primaryColor": "#21262d", "primaryTextColor": "#e6edf3", "primaryBorderColor": "#8b949e", "lineColor": "#8b949e", "textColor": "#e6edf3", "edgeLabelBackground": "#161b22", "actorBkg": "#21262d", "actorBorder": "#8b949e", "actorTextColor": "#e6edf3", "actorLineColor": "#8b949e", "signalColor": "#8b949e", "signalTextColor": "#e6edf3", "noteBkgColor": "#373320", "noteBorderColor": "#d4a72c", "noteTextColor": "#f0e6c0", "labelBoxBkgColor": "#21262d", "labelBoxBorderColor": "#8b949e", "labelTextColor": "#e6edf3", "loopTextColor": "#e6edf3", "activationBkgColor": "#30363d", "activationBorderColor": "#8b949e"}}}%%
sequenceDiagram
    participant User
    participant TabManager as TabManager (@MainActor)
    participant Publisher as selectedTabIdPublisher (CurrentValueSubject)
    participant TabItemView

    Note over User,TabItemView: Before fix — two render phases
    User->>TabManager: click workspace
    TabManager->>Publisher: send(newId) [synchronous, on main]
    Publisher-->>TabItemView: .receive(on: RunLoop.main) schedules next cycle
    TabItemView->>TabItemView: render phase 1: multi-selected (opacity 0.25)
    Note over TabItemView: next RunLoop cycle
    Publisher-->>TabItemView: "callback fires, observedIsActive = true"
    TabItemView->>TabItemView: render phase 2: solid active fill (opacity 1.0)

    Note over User,TabItemView: After fix — single render phase
    User->>TabManager: click workspace
    TabManager->>Publisher: send(newId) [synchronous, on main]
    Publisher-->>TabItemView: onReceive fires in same transaction
    TabItemView->>TabItemView: "observedIsActive = true, single solid render"

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_{Reviews (1): Last reviewed commit: "Fix sidebar tab selection highlight timi..." | Re-trigger Greptile}