[Bug]: MM extension not loading on Arc browser

[Unik0rnMaggie]

Describe the bug

When trying to load MetaMask extension on Arc browser, onboarding is not possible

Expected behavior

Be able to load MM extension on Arc browser

Screenshots/Recordings

Screen.Recording.2026-06-08.at.16.43.24.mov

Steps to reproduce

1. Install MM extension v 13.35.0 on Arc browser

Error messages or log output

Where was this bug found?

Internal release testing

Version

13.35.0

Build type

None

Browser

Other (please elaborate in the "Additional Context" section)

Operating system

MacOS

Hardware wallet

No response

Additional context

Arc Browser

Severity

No response

[mm-token-exchange-service]

Triage Agent -- Classification

Severity: Sev1 (confidence: 0.82) ⚠️ Advisory

MetaMask extension completely fails to load on Arc browser, preventing onboarding entirely. This is a total loss of functionality for all Arc browser users with no workaround mentioned. While Arc is not the most popular browser, it has a significant user base among Chromium-based browser users.
Not auto-applied -- confidence below threshold.

Size: size-S (confidence: 0.75) ⚠️ Advisory

The root cause is likely related to Arc browser's side panel support detection or environment type detection. The fix in getIsSidePanelFeatureEnabled already exists in the current codebase (Arc detection via CSS variable), suggesting this may already be partially addressed. The fix would be small and well-scoped, likely involving ensuring the side panel feature is properly disabled for Arc and that the onboarding flow handles this gracefully.
Not auto-applied -- confidence below threshold.

Team: team-onboarding ✅

Matched via area fallback: area-onboarding (no CODEOWNERS match)

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Root Cause Analysis

Affected component: Side panel feature detection and onboarding initialization on Arc browser

Arc browser is a Chromium-based browser that reports chrome.sidePanel as available (since it uses Chrome's extension APIs) but doesn't properly support the side panel functionality. When MetaMask detects side panel support and attempts to use it during onboarding, the extension fails to initialize properly on Arc. The getIsSidePanelFeatureEnabled() function in shared/lib/environment.ts now includes Arc-specific detection using the --arc-palette-title CSS variable, but this detection may not work during early startup (before the document is fully loaded) or the side panel detection may be called before the CSS variable is available. Additionally, the useSidePanelEnabled hook used in onboarding-flow.tsx and the ENVIRONMENT_TYPE_SIDEPANEL detection in environment-type.ts could cause issues if Arc opens the extension in a side panel context that doesn't work properly. The CriticalStartupErrorHandler shows that background connection timeouts (15 seconds) could also manifest as a blank screen if the side panel port connection fails silently on Arc.

Evidence:

• Arc browser detection code exists in getIsSidePanelFeatureEnabled() using --arc-palette-title CSS variable, indicating this is a known issue
• Comment in code: 'Arc browser doesn't support sidepanel properly'
• The onboarding flow uses useSidePanelEnabled hook and checks ENVIRONMENT_TYPE_SIDEPANEL which could behave incorrectly on Arc
• Reporter is on v13.35.0 - the Arc detection code may have been added after this version
• The getIsSidePanelFeatureEnabled checks chrome.sidePanel existence first, which would return true on Arc since it uses Chrome APIs

Suggested Fix

Confidence: medium: likely but needs investigation - the Arc detection code exists in the current codebase but may have been added after v13.35.0, or the CSS variable detection may have timing issues

The Arc browser detection in getIsSidePanelFeatureEnabled() appears to already exist in the current codebase. If the reporter is on v13.35.0 and this fix was added later, the issue may already be resolved. However, if the CSS variable detection is unreliable (e.g., timing issues during early startup), a more robust detection method may be needed. Additionally, ensure that when side panel is disabled, the onboarding flow gracefully falls back to fullscreen mode.

Target files: shared/lib/environment.ts, ui/pages/onboarding-flow/onboarding-flow.tsx
Target functions: getIsSidePanelFeatureEnabled, OnboardingFlow

Approach:

1. Verify that getIsSidePanelFeatureEnabled() is called early enough and that the --arc-palette-title CSS variable is available at that point. 2. If the CSS variable isn't available during early startup, consider adding a fallback detection method (e.g., checking navigator.userAgent patterns specific to Arc, or checking for other Arc-specific DOM properties). 3. Ensure the onboarding flow in onboarding-flow.tsx handles the case where isSidePanelEnabled returns false gracefully, particularly the setCompletedOnboardingWithSidepanel and setUseSidePanelAsDefault paths. 4. Consider adding a try-catch around the side panel initialization to prevent it from blocking the entire extension load.

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Confidence threshold: 0.85. Labels marked ✅ were auto-applied. Labels marked ⚠️ are advisory only (below threshold).
To re-triage, remove the ta-bot-triage label and re-apply ta-needs-triage.
⚠️ Some dimensions could not be confidently classified. An engineer should review this issue.

[Unik0rnMaggie]

Unable to reproduce on prod build v13.34.0:

Screen.Recording.2026-06-08.at.16.51.24.mov