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Merging two diverged codebases without regressions

Two React applications had forked years earlier and drifted — ~580 shared files duplicated or diverged. I planned and executed a staged merge (355 files moved, 339 deduplicated, ~245 reconciled with LLM-assisted review), then fixed all 13 launch blockers within a week.

When
Jul 2026
Role
Lead engineer
Context
Folding a separate product’s frontend into the main monorepo after a company-wide product merger

The problem

After a product merger, one product’s React app still lived in its own repository. Every shared component existed twice, fixes had to land twice, and the two copies had quietly drifted: about 580 shared files were duplicated or diverged. That blocked a unified build and a consistent experience.

Constraints

  • Both products were live; neither could regress.
  • The two apps ran different React major versions (18 vs 16) with different render semantics.
  • Separate i18n catalogs, Webpack entry points and global state.

Approach

Not a bulk copy — a staged blend where each stage is mechanically verifiable:

  1. Move the 355 files that existed only in the second app.
  2. Deduplicate the 339 byte-identical files.
  3. Reconcile the ~245 diverged shared files in reviewed batches — adopt the main-tree version where safe, graft the second app’s behavior where it wasn’t. I used LLM-assisted diff review to summarize each divergence, with a human review of every batch.
  4. Unify the i18n catalogs and wire the bundles.
  5. Port the second app’s code down to React 16 semantics — for example, a component returning undefined throws in 16 where 18 renders nothing.

Fallout

QA filed 13 launch blockers in one day. I root-caused and fixed all of them within a week: scoping bugs in a shared account picker, a notification tray that was never mounted, request races where the newest response must win, and React 16 render crashes.

Impact

One codebase and one bundle pipeline for both products; double maintenance of shared components ended.

What I’d do differently

Add lint rules for React-18-only patterns before the merge rather than after the crashes, so render-semantics bugs fail the build instead of reaching QA.