Guard workspace sidebar LazyVStack against layout re-livelock (#6384)

[austinywang]

Fixes #6384

Investigation

#6384 reports a ~1s main-thread beachball with many surfaces open: the main thread is 100% on-CPU (not blocked) inside GraphHost.flushTransactions(), recomputing layout for a ScrollView + LazyVStack + ForEach tree — the workspace sidebar.

The authoritative follow-up comment (from a second reporter on 0.64.16) pinpoints the cmux-owned frames as SidebarRowsFillLayout.placeSubviews / .sizeThatFits driving LazyStack → ForEachList.applyNodes → measureEstimates, and notes this is the force-measure root-caused in #6210 and removed in #6188 (merged 2026-06-16, ~14h after v0.64.16 was tagged — so the stable build both reporters hit still shipped it).

I confirmed on current main:

• The root cause is already fixed. 313a2d2 (Sidebar: remove rows measurement that re-livelocks layout at scale #6188) is an ancestor of HEAD; SidebarRowsFillLayout is deleted (only a historical comment remains), and there is no ProposedViewSize(… nil) / sizeThatFits(… nil) force-measure anywhere in the repo. The workspace sidebar (workspaceScrollContent/workspaceRows) is lazy again: LazyVStack + a measurement-free .frame(minHeight:) fed by a GeometryReader that sits outside the ScrollView and only supplies a concrete viewport-derived minHeight constant.
• Adjacent hardening is also in place: allocation-free row identity (SidebarWorkspaceRenderItemID enum, cmux.app 100% CPU hang: non-converging LazyHVStack layout loop over sidebar workspace list (SidebarWorkspaceRowIdsPreferenceKey feedback) — stable 0.64.14 #5764), a snapshot refresh policy that throttles re-renders, and a drag-only drop-anchor GeometryReader gated behind an active drag (Fix sidebar workspace frame collection virtualization #5325).
• The remaining trigger (per-frame spinner terminal-title updates dirtying the sidebar, Sidebar becomes unclickable when CLI tools emit frame-by-frame spinner characters in terminal titles #6291) is being handled by separate in-flight PRs (Collapse spinner-frame terminal titles to fix sidebar freeze (#6291) #6319, Fix sidebar freeze from spinner terminal titles (#6291) #6840, fix: dedupe terminal title updates to keep sidebar clickable during spinner output #6293) — out of scope here.

Because this is a layout/timing livelock that only manifests at agent-heavy scale on the reporter's stable build, it isn't cleanly reproducible locally or unit-testable as a SwiftUI layout pass; the structural fix already landed in #6188.

What this PR adds

This exact class of bug has regressed four times — #2586 → #5764 → #5845 → #6033/#6210/#6384 — and the lazy-at-measure-time contract is currently defended only by inline comments, which CI cannot enforce. This PR adds the missing CI regression guard so a re-introduction fails the build instead of shipping another beachball:

• scripts/check-sidebar-lazy-layout.py — neutralizes comments and string literals (the guarded functions deliberately name the forbidden anti-patterns in their explanatory comments), extracts the bodies of workspaceScrollContent and workspaceRows from Sources/ContentView.swift, and fails if either:
  • reintroduces a whole-list measurement signature: GeometryReader, ProposedViewSize(…, nil), .sizeThatFits(, or SidebarRowsFillLayout; or
  • drops a lazy-fill primitive the fix relies on: LazyVStack( in workspaceRows, .frame(minHeight:) in workspaceScrollContent.
  • A renamed/removed guarded function fails loudly (non-zero) rather than silently skipping, so the guard can't rot into a no-op.
• tests/test_ci_sidebar_lazy_layout_guard.py — proves the guard catches the bug: passes the real repo and a clean fixture whose comments/strings name every forbidden token, and fails synthetic fixtures for each regression mode (force-measure, reintroduced custom Layout, GeometryReader, eager VStack, missing minHeight, renamed function).
• Wires the self-test into the workflow-guard-tests CI job, matching the repo's existing guard-script convention (lint-pbxproj-test-wiring, check-workspace-package-groups, etc.).

The drag-only rowsWithGatedDropTargetReader is intentionally not scanned — its GeometryReader resolves per-row drop anchors and only runs during an active drag (#5325), never in the steady-state layout this guard protects.

Note on the two-commit regression policy

The red/green two-commit structure is for adding a new code fix. Here the fix already shipped in #6188, so the guard passes on main precisely because main is correct. The proof that the guard catches the bug is instead permanent and in-CI: the negative cases in test_ci_sidebar_lazy_layout_guard.py reintroduce each anti-pattern and assert the guard fails.

Validation

$ python3 scripts/check-sidebar-lazy-layout.py
check-sidebar-lazy-layout: ok (Sources/ContentView.swift)

$ python3 tests/test_ci_sidebar_lazy_layout_guard.py
[PASS] real repo passes
[PASS] clean body, anti-patterns only in comments/strings
[PASS] force-measure sizeThatFits(ProposedViewSize(height: nil)) fails
[PASS] reintroduced SidebarRowsFillLayout fails
[PASS] GeometryReader in scroll content fails
[PASS] eager VStack (no LazyVStack) fails
[PASS] missing .frame(minHeight:) fails
[PASS] renamed guarded function fails (no silent skip)
test_ci_sidebar_lazy_layout_guard: all cases passed

No app/runtime behavior changes (CI-only). No user-facing strings, settings, menus, shortcuts, docs, or web messages touched — localization audit: N/A.

🤖 Generated with Claude Code

---

Summary by cubic

Adds a CI guard to keep the workspace sidebar LazyVStack lazy at measure time and prevent reintroducing whole‑list measurement behind #6384. Now scans repo-owned Sources/ and Packages/ for any custom Layout used with the rows, handles Swift multi-line strings, and remains CI-only.

• New Features
  • Added scripts/check-sidebar-lazy-layout.py to scan workspaceScrollContent and workspaceRows in Sources/ContentView.swift; fails on GeometryReader, .sizeThatFits(, ProposedViewSize(..., nil), or any Layout-conforming type applied to the rows (discovered across repo-owned Sources/ and Packages/, excluding .build, .git, and vendor dirs); requires LazyVStack( and .frame(minHeight:); neutralizes comments, strings, and """..."""; fails loudly if the guarded functions are renamed.
  • Added tests/test_ci_sidebar_lazy_layout_guard.py with pass/fail fixtures, including a renamed custom Layout, a multi-line string with a bare ", and a discovery check that includes Packages/ and excludes vendored build trees; wired into CI via .github/workflows/ci.yml.

^{Written for commit 3cb88e6. Summary will update on new commits.}

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Project	Deployment	Actions	Updated (UTC)
cmux	Canceled		Jun 26, 2026 3:29pm
cmux-staging	Building	Preview, Comment	Jun 26, 2026 3:29pm

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📥 Commits

Reviewing files that changed from the base of the PR and between 6d6c701 and 3cb88e6.

📒 Files selected for processing (3)

• .github/workflows/ci.yml
• scripts/check-sidebar-lazy-layout.py
• tests/test_ci_sidebar_lazy_layout_guard.py

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch issue-6384-main-thread-hang-1s-beachball-in

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[greptile-apps]

Greptile Summary

This CI-only PR adds a regression guard to keep the workspace sidebar's LazyVStack lazy at measure time, preventing a re-introduction of the ~1s main-thread beachball (bug #6384) that was fixed in #6188 but has regressed four times. No runtime or user-facing changes are made.

• scripts/check-sidebar-lazy-layout.py scans workspaceScrollContent and workspaceRows in Sources/ContentView.swift for forbidden whole-list measurement patterns (GeometryReader, .sizeThatFits(, ProposedViewSize(…, nil), any custom Layout-conforming type) and verifies required lazy primitives (LazyVStack(, .frame(minHeight:)) are still present; it also fails loudly on guarded-function rename, closing the "rot into a no-op" escape hatch.
• tests/test_ci_sidebar_lazy_layout_guard.py exercises 9 positive/negative cases — including the newly added multi-line string and renamed-Layout cases — and the test is wired into the existing workflow-guard-tests CI job.

Confidence Score: 5/5

CI-only additions with no runtime changes; the guard logic is correct for all current and historically regressed patterns.

All three files are pure build tooling — no Swift sources, no runtime paths, no user-facing behavior is touched. The guard's core state machine (comment/string neutralization, brace-matched function extraction, forbidden-pattern scanning) is well-tested across 9 positive and negative cases. The two gaps identified (generic-constraint false positive in CUSTOM_LAYOUT_DECL and unhandled raw string literals) would, if triggered, produce visible CI noise rather than silent regressions, and neither is plausible in the guarded functions today.

No files require special attention for merge safety; the two noted edge cases in scripts/check-sidebar-lazy-layout.py are low-probability and would manifest as obvious CI false positives rather than missed real bugs.

Important Files Changed

Filename	Overview
.github/workflows/ci.yml	Adds a single step to run the sidebar lazy-layout guard self-test in the existing workflow-guard-tests job; matches the repo's established pattern for guard-script CI wiring.
scripts/check-sidebar-lazy-layout.py	Well-structured guard with comment/string neutralization (including the newly added multi-line-string state), brace-matched function extraction, and cross-file Layout-type discovery; two minor regex gaps — generic-constraint false positives in CUSTOM_LAYOUT_DECL and unhandled raw-string literals — could cause spurious CI failures in unlikely but possible future code patterns.
tests/test_ci_sidebar_lazy_layout_guard.py	Comprehensive self-test covering 9 pass/fail cases (real repo, clean comments/strings, force-measure, SidebarRowsFillLayout, GeometryReader, eager VStack, missing minHeight, renamed function, and cross-tree Layout discovery); the new multi-line string (b2) and renamed-Layout (d2/i) cases are well-exercised.

Flowchart

%%{init: {'theme': 'neutral'}}%%
flowchart TD
    A([check-sidebar-lazy-layout.py]) --> B[Read Sources/ContentView.swift]
    A --> C[Scan Sources/ + Packages/ for Layout-conforming types]
    B --> D[neutralize_swift: strip comments, strings, multi-line strings]
    C --> D2[neutralize_swift each file]
    D2 --> E[CUSTOM_LAYOUT_DECL regex → custom_layout_names set]
    D --> F[extract_function_body: workspaceScrollContent]
    D --> G[extract_function_body: workspaceRows]
    F --> F1{function found?}
    G --> G1{function found?}
    F1 -- No --> FAIL([FAIL: rename detected])
    G1 -- No --> FAIL
    F1 -- Yes --> H[Check FORBIDDEN_PATTERNS in body]
    G1 -- Yes --> H
    E --> H
    H --> H1{GeometryReader / sizeThatFits / ProposedViewSize nil / SidebarRowsFillLayout / any custom Layout name?}
    H1 -- Found --> FAIL
    H1 -- Clean --> I[Check REQUIRED_PRIMITIVES]
    I --> I1{LazyVStack in workspaceRows?}
    I1 -- Missing --> FAIL
    I1 -- Present --> I2{.frame minHeight in workspaceScrollContent?}
    I2 -- Missing --> FAIL
    I2 -- Present --> PASS([PASS: ok])

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"}}}%%
flowchart TD
    A([check-sidebar-lazy-layout.py]) --> B[Read Sources/ContentView.swift]
    A --> C[Scan Sources/ + Packages/ for Layout-conforming types]
    B --> D[neutralize_swift: strip comments, strings, multi-line strings]
    C --> D2[neutralize_swift each file]
    D2 --> E[CUSTOM_LAYOUT_DECL regex → custom_layout_names set]
    D --> F[extract_function_body: workspaceScrollContent]
    D --> G[extract_function_body: workspaceRows]
    F --> F1{function found?}
    G --> G1{function found?}
    F1 -- No --> FAIL([FAIL: rename detected])
    G1 -- No --> FAIL
    F1 -- Yes --> H[Check FORBIDDEN_PATTERNS in body]
    G1 -- Yes --> H
    E --> H
    H --> H1{GeometryReader / sizeThatFits / ProposedViewSize nil / SidebarRowsFillLayout / any custom Layout name?}
    H1 -- Found --> FAIL
    H1 -- Clean --> I[Check REQUIRED_PRIMITIVES]
    I --> I1{LazyVStack in workspaceRows?}
    I1 -- Missing --> FAIL
    I1 -- Present --> I2{.frame minHeight in workspaceScrollContent?}
    I2 -- Missing --> FAIL
    I2 -- Present --> PASS([PASS: ok])

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_{Reviews (4): Last reviewed commit: "Guard: discover custom Layouts across Pa..." | Re-trigger Greptile}

[austinywang]

Thanks @greptile — addressed the P2 in 73903a2: neutralize_swift now has a dedicated multi-line-string state (""" opens it, only a closing """ ends it, a lone " inside is neutralized), so a bare quote in a """...""" literal can no longer close the literal early and expose a prose-named forbidden token as code. Added self-test case (b2) covering a multi-line string that contains a bare quote plus GeometryReader / sizeThatFits(ProposedViewSize(height: nil)) / SidebarRowsFillLayout, asserting the guard still passes.

[greptile-apps]

The fix is solid. The MULTILINE_STRING state is correct: """ opens it, only a closing """ ends it, and a lone " inside is blitted as a space — exactly the right semantics. The test case (b2) exercises the exact failure mode (bare quote inside """...""" with forbidden tokens in prose) and asserts the guard still passes.

LGTM — this addresses the false-positive concern completely.