App breaks after browser update — how do you trace the issue?

When an app breaks right after a browser update, I assume nothing and trace it systematically. Browser updates usually expose standards violations, deprecated APIs, or timing issues that were already there.

Here’s the exact process I follow in real projects.

1. Reproduce on the Updated Browser (First 5 Minutes)

Never debug blind.

• Confirm browser version number
• Test in:
  • Normal mode
  • Incognito / Private mode
• Try with:
  • Cache disabled
  • Extensions disabled

If it works in Incognito → extension / cache issue.

2. Open DevTools → Console (Highest Signal)

Browser updates usually log clear warnings.

Look for:

• Deprecated API used
• TypeError after update
• Blocked by permissions policy
• Uncaught DOMException

Chrome, Safari, and Firefox now break hard instead of warning.

3. Check Browser Release Notes (Critical Step)

This saves hours.

Search:

Chrome 123 breaking changes
Safari 17 deprecated APIs
Firefox permissions policy update

Common breaking changes:

• Removed:
  • document.execCommand
  • webkit* APIs
  • Sync XHR on main thread
• Cookie changes:
  • SameSite
  • Secure enforcement
• Third-party cookies disabled
• Mixed content stricter blocking

4. Network Tab – Look for Silent Failures

Many apps “break” due to blocked requests.

Check:

• CORS failures
• OPTIONS preflight blocked
• Cookies not sent
• 401 / 403 after update

Especially common after:

• Chrome privacy updates
• Safari ITP changes

5. Identify Feature Detection vs User-Agent Sniffing

Browser updates often change UA strings.

Red flag:

if (navigator.userAgent.includes('Chrome')) { ... }

Fix:

Use feature detection:

if ('IntersectionObserver' in window) { ... }

6. CSS Regressions (Very Common)

Browser rendering engines change behavior.

First suspects:

• position: sticky
• flex + min-height
• overflow + vh
• z-index stacking context
• backdrop-filter

Use Computed Styles tab to compare old vs new.

7. Third-Party Libraries

Browser updates break apps indirectly.

Check:

• Is a CDN serving a newer build?
• Is the library using deprecated APIs?
• Was the bundle transpiled correctly?

Lock versions where possible.

8. Storage & Cookies

Modern browsers tighten security frequently.

Common failures:

• Cookies without SameSite=None; Secure
• localStorage blocked in 3rd-party context
• IndexedDB version upgrade failures

Check Application / Storage tab.

9. Disable Parts of the App (Binary Search)

Fastest way to isolate.

• Comment out:
  • New JS modules
  • New CSS
• Replace API calls with mock data
• Re-enable step by step

You’ll hit the breaking line quickly.

10. Compare Against Another Browser

If it still works in Firefox/Edge:

• It’s standards compliance
• Or a browser-specific change

Check:

• caniuse.com
• MDN deprecation notices

What Usually Breaks After Updates (Real-World List)

• Date parsing (new Date('YYYY-MM-DD HH:mm'))
• Synchronous XHR
• Cross-site cookies
• Unprefixed CSS
• Old polyfills interfering with native APIs
• Promise timing / microtask order assumptions

TL;DR – My Exact Order

• Reproduce + Console errors
• Read browser release notes
• Network & cookies
• CSS layout changes
• Third-party libs
• Binary isolation