fix(checkout): preserve full billing address when country changes

[detail-app]

Detail bug report: View on Detail

Summary

Related Issue: polarsource/feedback#417

When a user changes the billing country dropdown in the checkout form, the watcher sent only { country: newCountry } to the server instead of the full billing address. The server replaces the entire customer_billing_address on update, so all other fields (line1, city, postal_code, state) were dropped from server state — causing incorrect tax calculations and address data loss on page refresh.

What

• CheckoutForm.tsx: The country-change watcher now spreads ...customerBillingAddress (all existing form address fields) and overrides country: newCountry, mirroring the pattern already used by the other-address-field watcher branch. When the state field is reset (switching between fixed-state countries like US/CA), state: '' is included in the payload so the server receives the reset value consistent with the form UI.
• CheckoutForm.test.tsx: Updated the existing "resets state field when country changes" test to assert the update() payload includes all address fields (not just country). Added a new test verifying all fields are sent when changing between two non-fixed-state countries (no state reset).

Why

The country-change watcher was inconsistent with the other address field watchers: the country path sent only { country } while the other fields spread the full customerBillingAddress. Because the server does a full replace (checkout.customer_billing_address = checkout_update.customer_billing_address), sending only country truncated the stored address. This caused:

• Incorrect tax — tax APIs (Stripe Tax / Numeral) received only the country, missing postal_code/state/city needed for jurisdiction-accurate rates.
• Address loss on refresh — after a page refresh, the form re-initialized from the truncated server state, losing all address fields except country.

The bug was introduced in ba33eac70d (PR #4060), which added the watcher pattern for address field changes.

How

The fix is a single client-side change: spread customerBillingAddress in the country-change payload, the same way the other-address-field branch already does. No server-side changes are needed — the server's full-replace semantics are correct when the client sends the full address. The shouldResetState condition was extracted into a variable to avoid duplicating the state-reset check between the resetField call and the payload construction.

Testing

• Unit tests: All 31 CheckoutForm tests pass, including the updated "resets state field when country changes" (now asserts the PATCH payload contains all address fields with state: '') and the new "sends all billing address fields when country changes without state reset" (verifies fields are preserved when switching SE → DE). The full checkout package suite (389 tests) passes with no regressions.
• Typecheck and format: tsc --noEmit and oxfmt --check both pass clean.
• Server-side tests: The full checkout test suite (456 tests) passes, confirming the server's address-replacement and tax-calculation flow works correctly with the full payload — no server changes were needed.
• E2E (browser): Drove a live checkout with Playwright against the local stack (API server + Next.js web app). Filled billing address fields, changed the country dropdown, and verified: (1) the PATCH request sent to the server included the full address (line1, postal_code, city, state, country), (2) after page refresh the form fields were still populated from server state, and (3) the server API confirmed the full address was stored. Also tested with a paid product — the client correctly sent the full address on both US and FR country changes (confirmed via request capture); server-side persistence couldn't be fully verified for the paid product because the dev Stripe key is a placeholder, but a direct API test on the free product confirmed the server stores and returns the full address on country change.

Checklist

• This PR addresses a single concern (one bug fix, one feature, one refactor)
• The diff is reasonably sized and easy to review
• New functionality is covered by tests
• Linting and type checking pass (uv run task lint && uv run task lint_types)
• No unrelated changes or drive-by fixes are included

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Project	Deployment	Actions	Updated (UTC)
orbit	Ready	Preview	Aug 29, 2026 12:36am
polar-test	Ready	Preview	Aug 29, 2026 12:36am