A mirrored tmux window with one pane shows two tab bars

[ejc3]

Mirroring a remote tmux window put two tab bars on top of each other. The workspace tabs run across the top — one per tmux window — and directly beneath them sat a pane tab bar holding a single tab with the same title as the tab above it.

┌─ main ─────────────────────────────────────────────┐
│ [ playwright ×] [ excalidraw ] [ fb4a_sim ] [ zsh ] │   workspace tabs (tmux windows)
│ [ playwright ×]                                     │   pane tab bar — one tab, same name
├─────────────────────────────────────────────────────┤
│ $                                                   │

Every pane draws its own tab bar, and that bar exists so you can switch between the tabs open in that pane. A mirrored pane only ever has one tab: it shows exactly one remote tmux pane, and nothing else can be opened inside it.

Splitting does not change that. It adds a second pane, which brings its own one-tab bar — two bars on screen, one tab each, not one bar with two tabs. So the number of bars follows the pane count while the tabs inside any one bar stay at one.

With a single pane that leaves a bar with nothing to switch between, drawing one tab whose title repeats the workspace tab directly above it.

Visibility now follows the window's pane count. At one pane the bar is hidden. Split, and the per-pane bars appear — not because there is suddenly something to switch between, but because there is now something to identify: each bar names its pane and carries that pane's close and split buttons. The buttons for splitting a single-pane window live in the workspace tab row, so nothing becomes unreachable.

The rule is derived in the layout reconcile rather than at the split and close call sites. tmux changes the pane count without either one — a pane exiting on its own, or a kill-pane from another client — and every one of those arrives as a layout, so reconcile is the only place that sees them all.

.multipleTabs was not usable for this. It keys off the tabs inside a pane, which is the count that never reaches two here, so it would have hidden the bars with four panes on screen just as readily as with one.

The sizing half

Hiding the bar frees the height it occupied. The sizing calculation was charging appearance.tabBarHeight for it regardless of whether it was drawn. Left alone it would have kept claiming a grid sized for chrome that is no longer drawn, so the pane would have asked tmux for fewer rows than it had room for and stranded a strip at the bottom that nothing renders into. It now asks the same visibility rule, and a change in visibility re-arms the sizing pass exactly as a change in tab bar height already does.

Tests

Two commits, so the tests are visibly red before the fix:

• a single-pane mirror hides its pane tab bar
• splitting shows the bars, and closing back down to one pane hides them again — driven by publishing layouts, which is how every real pane-count change arrives
• a single-pane mirror claims the rows its hidden bar frees (36 instead of 34 on the suite's calibrated 620pt container), with the visibility premise asserted in the same test so a change to the rule surfaces there rather than as silent row drift

Against the old behavior the first two fail with (tabBarVisibility → .always) == .multipleTabs, and the sizing one fails on the row count. The two-pane assertion passes either way, which is correct — that case does not change.

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Summary by cubic

A mirrored tmux window with one pane no longer shows a duplicate pane tab bar beneath the workspace tabs; split windows still show per-pane bars. Hidden bars no longer reserve terminal rows, so the mirror uses the full container height.

Bug Fixes

• Derives visibility from pane count during layout reconciliation, including panes that exit or are killed externally.
• Re-runs sizing when pane count changes and applies the same visibility rule to the height calculation.
• Adds tests for single-pane hiding, split-and-close transitions, and reclaimed rows.

^{Written for commit e6fe1f8. Summary will update on new commits.}

Summary by CodeRabbit

• Bug Fixes

  • Corrected pane tab bar visibility so single-pane windows hide the bar, while multi-pane windows display it.
  • Updated layout sizing to avoid reserving space for hidden tab bars, giving single-pane content the space previously taken by the bar.
  • Kept visibility synchronized when panes are split, closed, or removed externally, ensuring content uses the available space correctly.

• Tests

  • Added coverage for tab bar transitions and accurate single-pane content sizing.

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📥 Commits

Reviewing files that changed from the base of the PR and between ebda598 and 1a5cc0d.

📒 Files selected for processing (4)

• Sources/RemoteTmuxWindowMirror+BonsplitLayout.swift
• Sources/RemoteTmuxWindowMirror.swift
• cmux.xcodeproj/project.pbxproj
• cmuxTests/RemoteTmuxMirrorFeedForwardTests.swift

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📝 Walkthrough

Walkthrough

The mirror derives pane tab bar visibility from pane count. Single-pane layouts hide the bar and exclude its height from sizing metrics. Multi-pane layouts show the bar. Tests cover layout transitions and row calculations.

Changes

Pane tab bar visibility

Layer / File(s)	Summary
Pane count visibility configuration Sources/RemoteTmuxWindowMirror+Configuration.swift	Maps pane counts to tab bar visibility and applies the result during Bonsplit configuration.
Layout reconciliation and sizing synchronization Sources/RemoteTmuxWindowMirror.swift, Sources/RemoteTmuxWindowMirror+Configuration.swift, Sources/RemoteTmuxWindowMirror+BonsplitLayout.swift	Recomputes visibility after layout reconciliation and charges tab bar height only when the bar is shown.
Visibility and geometry test coverage cmuxTests/RemoteTmuxMirrorPaneTabBarVisibilityTests.swift, cmuxTests/RemoteTmuxMirrorFeedForwardTests.swift, cmux.xcodeproj/project.pbxproj	Tests visibility transitions and single-pane row sizing. Registers the new test source in the Xcode project.

Priority: ⬇️ Low

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

Change: Bug fix

Merge Risk: ⚪ Minimal · up to 1a5cc

The change hides pane bars for single-pane mirrored windows and avoids reserving their height. The supplied tests cover the intended transitions and sizing behavior, with no concrete merge-blocking risk evident.

Security Architecture Review

Security architecture risk: 🔵 Low · up to 1a5cc

The change affects tab visibility and remote window sizing, but the reviewed paths do not show new access or authority. Security coverage is incomplete, so this is a low-risk assessment rather than a claim of no risk.

Retained concerns
No architecture-level concerns identified.

Security review details

Security Blast Radius

• inferred — The changed calculation can affect the size claimed for a mirrored tmux window, but the reviewed path retains the mirror’s existing per-window sizing authority rather than adding a new caller or privileged sink.

Trust Boundaries and Controls

• observed — Pending remote layouts remain subject to rectangle-coverage verification before publication, so the new visibility derivation consumes the established verified-layout path.

Resilience and Maintainability Implications

• observed — The sizing path coalesces follow-up work, while the existing hidden-window guard prevents a later hidden pass from overwriting an established size claim.

🚥 Pre-merge checks | ✅ 23 | ❌ 2

❌ Failed checks (2 warnings)

Check name	Status	Explanation	Resolution
Description check	⚠️ Warning	The description clearly explains the problem, resulting behavior, implementation, and test coverage. It does not include the required Demo Video or screenshots, does not state the test commands and ex…	Add a short demo video or screenshots. Document the exact test command or CI lane, what passed, and any unverified items. Restore the checklist and mark each applicable item or explain why it does not apply.
Docstring Coverage	⚠️ Warning	Docstring coverage is 60.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 10 functions across 5 files. (1 skipped: …	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (23 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title is concise, specific, and directly describes the duplicate tab-bar problem addressed by the pull request.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Cmux Cloud Persistent Session And Early Input	✅ Passed	PASS. The authoritative PR diff changes only RemoteTmux pane tab-bar visibility, layout sizing, reconciliation, project registration, and tests. It does not add or alter Cloud terminal creation, cmux-…
Cmux Swift Actor Isolation	✅ Passed	The production diff does not introduce a Swift actor-isolation mistake. RemoteTmuxWindowMirror is already @MainActor, and the changed instance methods remain on that actor. The new pure pane-count…
Cmux Swift Blocking Runtime	✅ Passed	PASS — The production diff adds no semaphore, blocking wait, sleep, timer, polling loop, main-queue sync, or manual lock. It only derives tab-bar visibility and calls the existing coalesced `setNeedsS…
Cmux Browser Automation Off-Main	✅ Passed	PASS: The PR changes only RemoteTmux mirror layout/configuration code, project registration, and RemoteTmux tests. It does not modify Sources/TerminalController.swift or `ControlCommandExecutionPoli…
Cmux Expensive Synchronous Load	✅ Passed	The authoritative diff changes only Bonsplit tab-bar visibility, layout metrics, and pane-count reconciliation. Although the configuration extension is @MainActor and reconciliation is an interactive …
Cmux Cache Substitution Correctness	✅ Passed	The production diff changes only mirrored-window UI configuration and sizing. It derives tab-bar visibility from the current layout pane count and reads the current Bonsplit configuration; it does not…
Cmux No Hacky Sleeps	✅ Passed	PASS: The pull request changes only Swift source/tests and an Xcode project file. It introduces no TypeScript, JavaScript, shell, or non-Swift build/runtime script changes. The authoritative patch als…
Cmux Algorithmic Complexity	✅ Passed	PASS. The production changes add constant-time visibility checks and a pane-count comparison. paneIDsInOrder.count performs a linear traversal of the tmux pane tree, but it is not nested, sorted, fi…
Cmux Swift Concurrency	✅ Passed	The pull request adds only synchronous tab-bar visibility and sizing logic, plus a synchronous reconcile call. The changed production diff adds no background Dispatch queues, Combine state, completion…
Cmux Swift @Concurrent	✅ Passed	The Swift diff does not introduce a concurrency violation. The new paneTabBarVisibility(paneCount:) is nonisolated but synchronous and pure, which the rule explicitly allows. `updatePaneTabBarVisi…
Cmux Swift Package Boundaries	✅ Passed	The PR does not introduce a package-boundary violation. The changed code remains app-specific RemoteTmuxWindowMirror composition: the mirror is @MainActor, imports AppKit, owns `BonsplitController…
Cmux Swiftpm Lockfiles	✅ Passed	PASS. The PR changes Swift sources, tests, and registers one test file in cmux.xcodeproj/project.pbxproj. It does not change Package.swift, Package.resolved, .gitignore, workflows, or Xcode Sw…
Cmux Swift Logging	✅ Passed	The PR adds no production Swift logging. The changed runtime files add only tab-bar visibility and sizing logic; the diff contains no print, debugPrint, dump, NSLog, ad hoc file/stdout logging…
Cmux User-Facing Error Privacy	✅ Passed	PASS. The production diff changes pane tab-bar visibility and sizing only. It adds no user-facing errors, alerts, command output, API error bodies, or recovery copy. The added explanatory text is in d…
Cmux Full Internationalization	✅ Passed	The production diff changes pane tab-bar visibility and sizing only. It adds no user-facing Swift text, localization key, web message, catalog entry, or locale data. Added prose is developer-only comm…
Cmux Swiftui State Layout	✅ Passed	PASS — The PR does not introduce a prohibited SwiftUI state or layout pattern. The production diff changes the @MainActor RemoteTmuxWindowMirror Bonsplit controller and layout reconciliation, with no …
Cmux Architecture Rethink	✅ Passed	PASS. The change is a small local correctness fix with a clear invariant: pane-bar visibility derives from the mirrored layout's pane count. reconcile is the authoritative transition path, and the p…
Cmux Swift Auxiliary Window Close Shortcuts	✅ Passed	PASS. The PR changes remote tmux mirror layout, pane tab-bar visibility, sizing, and test registration only. The changed Swift files add no NSWindow, NSPanel, NSWindowController, SwiftUI Window, Windo…
Cmux Source Artifacts	✅ Passed	All six changed paths are hand-written Swift source, test source, or Xcode project configuration. The new test is registered as a normal test target source. No logs, screenshots, recordings, caches, t…
Cmux No Test Or Debug Seam In Production Source	✅ Passed	The production diff adds no #if DEBUG or test-build guard, no test/debug-named member, and no visibility widening or wrapper accessor. The new paneTabBarVisibility helper and `updatePaneTabBarVisi…

Full details: Description check

Explanation

The description clearly explains the problem, resulting behavior, implementation, and test coverage. It does not include the required Demo Video or screenshots, does not state the test commands and execution results, and omits the repository checklist.

Full details: Docstring Coverage

Explanation

Docstring coverage is 60.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 10 functions across 5 files. (1 skipped: 1 unsupported.)

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[ejc3]

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Actionable comments posted: 1

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Inline comments:
In `@Sources/RemoteTmuxWindowMirror`+Configuration.swift:
- Around line 26-27: Mark the pure paneTabBarVisibility(paneCount:) helper as
nonisolated static within the `@MainActor` extension, leaving its existing mapping
logic unchanged.

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: ASSERTIVE

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Run ID: 902f874a-d6a6-4a88-814c-3053c02b58c7

📥 Commits

Reviewing files that changed from the base of the PR and between ab6608c and 1515dd5.

📒 Files selected for processing (6)

• Sources/RemoteTmuxWindowMirror+BonsplitLayout.swift
• Sources/RemoteTmuxWindowMirror+Configuration.swift
• Sources/RemoteTmuxWindowMirror.swift
• cmux.xcodeproj/project.pbxproj
• cmuxTests/RemoteTmuxMirrorFeedForwardTests.swift
• cmuxTests/RemoteTmuxMirrorPaneTabBarVisibilityTests.swift

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[teamleaderleo]

This one's green and approved, thank you @ejc3!! Main moved underneath it, so I'm resolving the conflict and landing it shortly :)

[teamleaderleo]

Merged for real now, thank you @ejc3!! A mirrored tmux window with one pane no longer shows a second tab bar, and the terminal gets that space back. :D

[github-actions]

Merge receipt for e6fe1f8430: every check was green at merge (13 verified; 14 skipped by policy). Full suite runs on main after merge.