perf(web): avoid scanning chat history for sidebar backgrounds

[juliusmarminge]

Every chat DOM update invalidated the outer sidebar wrapper's :has([data-variant=inset]) rule, even though the app uses a normal sidebar. Remove that unused query. Inset compositions can set bg-sidebar explicitly through the existing className prop; current app callers and their backgrounds are unchanged.

Six counterbalanced production-browser rounds against the previous stack tip, now merged as main:

Median	Before	After
Cached thread switch	163.975 ms	104.425 ms
Growing code update	104.85 ms	27.05 ms
Switches with sampled blank frames	0/48	0/48

Switching improves 36.3% and growing-code synchronous updates 74.2%, each improving in all six paired rounds. Prose-streaming differences were inconsistent; no reliable prose gain is claimed. These are client rendering measurements on this runner, not provider throughput or overall app speed.

Methods, rejected experiments and exact revisions · Raw samples and scripts

Before:

Before switching recording

After:

After switching recording

Production build and targeted formatting/lint completed. Lint retains the unrelated existing Math.random-in-render warning. Browser checks covered light/dark, wide/narrow layouts, sidebar toggling, and interrupted navigation. Desktop shares this CSS; Electron was not run. Native mobile is unaffected.

GPT-6 / Codex, with agent-browser verification.

[macroscopeapp]

Approvability

Verdict: Not approved

Macroscope's review found this PR not approvable — This one-file change narrows the shared SidebarProvider's default background behavior for inset layouts, even though the current application caller supplies the background explicitly. Because it changes a product UI default, the change warrants human review.

^{You can add or adjust custom eligibility rules. Learn more.}

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Team

Run ID: 93598a39-f2ea-45da-8e32-a8d21a5658e9

📥 Commits

Reviewing files that changed from the base of the PR and between 211618f and 6a961ad.

📒 Files selected for processing (1)

• apps/web/src/components/ui/sidebar.tsx

Included review availability: Your plan provides up to 10 included reviews per hour; 9 remain after this review.

---

📝 Walkthrough

Walkthrough

The sidebar wrapper no longer applies bg-sidebar for inset variants. Inset layouts must provide their background class separately.

Changes

Sidebar background handling

Layer / File(s)	Summary
Remove wrapper background class apps/web/src/components/ui/sidebar.tsx	The wrapper keeps its base layout classes and caller-provided className. The conditional inset background class was removed.

Priority: ⬇️ Low

Estimated code review effort: 1 (Trivial) | ~5 minutes

Change: Bug fix

Suggested reviewers: t3dotgg

Merge Risk: ⚪ Minimal · up to 6a961

The styling change preserves the inset sidebar background while avoiding the unnecessary wrapper behavior, so no merge-blocking risk remains.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 1 functions across 1 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
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.
Title check	✅ Passed	The title clearly identifies the main change: preventing unnecessary chat-history scans caused by sidebar background styling.
Description check	✅ Passed	The description clearly explains the change, motivation, measured results, validation scope, and UI evidence. It does not use the template headings or checklist, but it contains the required informati…

• Fix all pre-merge checks with AI

✨ Finishing Touches 💡 1

📝 Generate docstrings 💡

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch legend-perf/sidebar-style-scope

---

_{Comment @coderabbitai help to get the list of available commands.}

[github-actions]

Thread transfer impact

✅ Thread transfer remains within every enforced ceiling.

Provider	Metric	Main baseline	This PR	Impact	PR ceiling
Codex	Total thread wire	13.6 KiB	13.6 KiB	−4 B (−0.0%)	15.1 KiB	✅
Codex	Thread snapshot wire	7.1 KiB	7.0 KiB	−4 B (−0.1%)	7.3 KiB	✅
Codex	Live turn WebSocket wire	6.6 KiB	6.6 KiB	0 B (0.0%)	7.8 KiB	✅
Codex	Live turn WebSocket decoded	57.1 KiB	57.1 KiB	0 B (0.0%)	66.4 KiB	✅
Codex	Live turn messages	10	10	0 (0.0%)	21	✅
Claude	Total thread wire	13.6 KiB	13.5 KiB	−14 B (−0.1%)	15.1 KiB	✅
Claude	Thread snapshot wire	7.1 KiB	7.1 KiB	−7 B (−0.1%)	7.3 KiB	✅
Claude	Live turn WebSocket wire	6.5 KiB	6.5 KiB	−7 B (−0.1%)	7.8 KiB	✅
Claude	Live turn WebSocket decoded	57.8 KiB	57.8 KiB	0 B (0.0%)	66.4 KiB	✅
Claude	Live turn messages	8	8	0 (0.0%)	21	✅

Baseline: 211618f · PR result: 6a961ad · Source CI: success

Scenario and decoded snapshot size

10 historical turns, 5 command tools per turn, 878.9 KiB retained MCP result per historical turn, and a 1.05 MiB retained result in the measured turn.

• Codex decoded thread snapshot: 113.9 KiB
• Claude decoded thread snapshot: 114.6 KiB

Updated in place by a trusted workflow. PR artifacts are strictly validated and never executed.