[Bug] JMD currency precision loss — users lose up to J$1.10 per transaction due to float truncation in convertMoneyAmount()

[islandbitcoin]

Unified tracking issue for JMD currency precision failures (Mobile)

GitHub Issue: #555

14 distinct bugs across backend + mobile. This issue tracks the mobile fixes.

Supersedes: ENG-81, ENG-87

Depends on: ENG-132 (backend fixes must land first for Wave 1)

PR Review: Nick must approve before merge. Dread (islandbitcoin) final sign-off.

[linear]

ENG-133

[islandbitcoin]

Companion backend issue: lnflash/flash#282

[forge0x]

🐛 Bug 1 Reproduced: convertMoneyAmount() Float Precision Loss

📹 Video Evidence

Side-by-side production recording (iOS sender + Android receiver)

Setup

• Sender: forgetest1 (iOS sim, JMD wallet)
• Receiver: forgetest2 (Android emu, JMD display)
• Tested on: Test env + Production

Results

Test	Sender	Receiver	Lost
Test 1 (test env, J$7)	-J$7.00 / -₿67	J$6.99 / ₿67	J$0.01
Test 2 (prod, J$1,234.56)	-J$1,234.56 / -₿11,811	J$1,233.85 / ₿11,805	J$0.71 + 6 sats
Test 3 (prod, J$1,234.56)	-J$1,234.56 / -₿11,812	J$1,233.46 / ₿11,797	J$1.10 + 15 sats

Root Cause

convertMoneyAmount() in app/hooks/use-price-conversion.ts (lines 74-81) only applies Math.round() for BTC amounts, not for USD/DisplayCurrency. When sats are reconverted to JMD, fractional results get truncated instead of rounded.

Additional Findings

• Balance proof: After multiple J$7 sends, receiver balance was J$24.99 instead of expected J$25.00
• Bug 2 also confirmed: Transaction list reconverts amounts at current price, masking historical discrepancies — only the balance reveals the precision leak
• Worst case observed: J$1.10 lost on a single J$1,234.56 transaction (0.09%)
• The bug is rate-timing dependent — manifests whenever sats↔JMD conversion produces a fractional result

Fix

Add Math.round() to the fiat conversion path in convertMoneyAmount(), matching the existing BTC path.

[forge0x]

🐛 Bug 2 Reproduced: Transaction detail screen ignores display currency

📹 Video Evidence

Transaction list → detail discrepancy (Android, JMD user)

The Bug

A JMD user sees two completely different amounts for the same transaction:

View	Amount	Currency
Transaction list	J$1,240.08	JMD (user display currency)
Transaction detail	$8.01	USD (raw settlement amount)

The user taps a J$1,240 transaction and sees $8.01 — no explanation, no JMD equivalent shown.

Root Cause

Two issues in app/components/transaction-item/TxItem.tsx (lines 54-65):

1. List view reconverts at current BTC price via moneyAmountToDisplayCurrencyString() instead of using tx.settlementDisplayAmount (the historical display amount set at settlement time)
2. Detail screen (transaction-detail-screen.tsx) shows raw settlementAmount in USD without converting to the user's display currency

Impact

• JMD users see inconsistent amounts between list and detail views
• List amounts drift over time as BTC price changes — a J$1,234 transaction today might show as J$1,250 tomorrow
• Detail screen shows USD with no JMD context, confusing for Jamaican users

Fix

• TxItem.tsx: Use tx.settlementDisplayAmount + tx.settlementDisplayCurrency for the primary amount instead of reconverting via current price
• transaction-detail-screen.tsx: Convert header amount to user's display currency

[forge0x]

🐛 Bug 5 Reproduced: Transaction labels say "USD" + detail screen shows raw USD for JMD users

📹 Video Evidence

Side-by-side iOS + Android recording

The Bug (3 issues in one)

5a. Wrong transaction labels: All transactions labeled "Sent USD" / "Received USD" even though the user's display currency is JMD. Should say "Sent" or "Sent JMD".

5b. Detail screen shows raw USD: Tapping a transaction that shows J$1,237 in the list opens a detail view showing $8.01 USD — the raw Ibex settlement amount with zero JMD context. Completely confusing for Jamaican users.

5c. List amounts reconvert at current BTC price: The tx list shows J$1,237.09 now, but the original send was J$1,234.56. The amount drifts as BTC/JMD price changes because TxItem.tsx reconverts sats → JMD at the current rate instead of using the historical settlementDisplayAmount.

Root Cause

Backend (get-transactions-for-wallet.ts lines ~37-50):

• displayCurrency: "USD" hardcoded — backend stamps every tx as USD regardless of user setting
• offset: 0n — price scaling completely broken (should be SAT_PRICE_PRECISION_OFFSET = 12)
• Math.floor(trx.exchangeRateCurrencySats) — truncates fractional sat prices to 0
• settlementDisplayAmount: \${trx.amount}`` — raw Ibex USD amount, never converted to JMD

Mobile (TxItem.tsx lines 54-65):

• Reconverts at current BTC price via moneyAmountToDisplayCurrencyString() instead of using tx.settlementDisplayAmount

Mobile (transaction-detail-screen.tsx):

• Shows raw settlementAmount in USD without converting to user's display currency

Fix

• Backend: Pass account displayCurrency into the adaptor, use correct offset, use Math.round not Math.floor, convert settlementDisplayAmount to user's currency
• Mobile TxItem: Use tx.settlementDisplayAmount + tx.settlementDisplayCurrency for primary amount
• Mobile detail screen: Convert header amount to user's display currency

[forge0x]

🔍 Bug 4 & Bug 7: Confirmed via code review (not visually reproducible with test setup)

Bug 4: BtcWallet.balance returns object instead of number

File: src/graphql/shared/types/object/btc-wallet.ts line 53

// BtcWallet — BROKEN: returns USDAmount class instance
return balanceSats  // ← raw object, serializes as {} in GraphQL

// UsdWallet — CORRECT: extracts numeric value
return Number(balance.asCents(8))

BTC wallet enabled on forgetest1 (iOS) — shows ₿0, which masks the bug since {} and 0 both display as zero. Would need a funded BTC wallet to demonstrate. The FractionalCentAmount scalar has no serialize method, so a non-zero USDAmount object would serialize as {} or throw.

Bug 7: Breez SDK hardcodes USD display currency

File: app/hooks/useBreezPayments.ts line 48

settlementDisplayCurrency: "USD",  // ← hardcoded, ignores user display currency

All Breez-settled transactions (non-custodial Lightning via BTC wallet) will show USD regardless of user's JMD setting. Cannot reproduce without funding the BTC wallet and making a Breez payment.

Status

Both bugs are confirmed in code but require funded BTC wallets / Breez transactions to demonstrate visually. The fix for both is straightforward:

• Bug 4: return Number(balanceSats.asCents(8)) (match UsdWallet pattern)
• Bug 7: Use displayCurrency from user's account settings instead of hardcoded "USD"

[forge0x]

Bug 7 — Breez SDK transactions show USD instead of JMD (video evidence)

Video: bug7-ios.mp4

What the video shows

1. Home screen: JMD user (forgetest1), Bitcoin balance ₿4,766 sats, recent swap tx shows ~J$501 JMD ✅
2. Tap swap transaction → detail screen shows $3.21 USD ❌
3. The same transaction displays as JMD on the home screen but USD in the detail view

Root cause

useBreezPayments.ts line 48 hardcodes:

settlementDisplayCurrency: "USD"

Should use the user's display currency preference instead.

Expected behavior

Transaction detail should show ~J$500 JMD (matching the home screen display), not $3.21 USD.