refactor: clear three SonarQube smells, and pin what one of them exposed

schedule1/index.tsx L414/L422 — two three-deep nested ternaries extracted into
named helpers with early returns. Sonar is right that these were confusing, and
in a specific way: they hid that `derived` has the OPPOSITE precedence in the two
figures. For cost, the mirror changes exactly one of three branches (only 140);
for the rate, it supersedes every row including 139 and 140. Same two codes,
opposite answer — legitimately, because deriveSchedule1 computes a rate for 139
(its volume is user-entered) but no cost for it (the cost is a Schedule 3 pull).
Now stated in a comment instead of implied by nesting depth.

That refactor turned up a coverage gap: inverting the rate precedence left all
268 schedule1 tests green, so the rule was pinned by nothing. Added a test that
asserts 139's rate moves with an entered volume while its cost stays the
Schedule 3 pull — mutation-verified, it fails on the inversion.

schedule5SubPage/derived.ts L71 — `stampedVolume ?? null` becomes a defaulted
`volume` parameter. Equivalent: a default fires only on `undefined`, and every
call site passing an explicit `null` already wanted `null`.

No behaviour change intended in any of the three. Suite 1291/1293 (two timeout
flakes, both pass in isolation), zero unhandled errors, build clean, and zero
type errors in the touched files against an origin/main baseline.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Author: Rylan-cgi

frontend/src/components/schedule1/__tests__/Schedule1.test.tsx

@@ -171,6 +171,34 @@ describe('Schedule1 editable page', () => {
     expect(costOf(SUBTOTAL)).toBe('674,000')
   })
 
+  test("139's RATE is mirrored while its COST stays the Schedule 3 pull (#291)", async () => {
+    // Row 139's two halves come from different places, and the page treats them differently on
+    // purpose: the cost is pulled from Schedule 3 and nothing on this page feeds it, but the VOLUME is
+    // user-entered -- so `deriveSchedule1` computes a rate for 139 (derived.ts:98) even though it
+    // computes no cost for it. The mirror therefore supersedes 139's rate and must NOT touch its cost.
+    //
+    // Written after a SonarQube refactor of a three-deep ternary (2026-08-21) exposed that this
+    // opposite-precedence rule was pinned by no test: inverting it left all 268 green.
+    server.use(http.get(URL, () => HttpResponse.json(schedule1Doc)))
+    render(<Schedule1 />)
+    const user = userEvent.setup()
+
+    const label = 'Less Silviculture Admin Costs'
+    // Served state: 150,000 pulled from Schedule 3 over the fixture's volume of 55.
+    expect(await screen.findByLabelText(`${label} volume`)).toHaveValue('55')
+    expect(rate(label)).toBe('2,727.27') // 150,000 / 55
+
+    const volume = screen.getByLabelText(`${label} volume`)
+    await user.clear(volume)
+    await user.type(volume, '60000')
+    await user.tab()
+
+    // The rate moved off the served figure -- the mirror owns it.
+    expect(rate(label)).toBe('2.50') // 150,000 / 60,000
+    // ...while the cost is still the Schedule 3 pull, untouched by the mirror.
+    expect(costOf(label)).toBe('150,000')
+  })
+
   test('the Other Costs $/m³ tracks the volume entered on this page (#291)', async () => {
     server.use(http.get(URL, () => HttpResponse.json(schedule1Doc)))
     render(<Schedule1 />)

frontend/src/components/schedule1/index.tsx

@@ -401,29 +401,37 @@ const Schedule1: FC = () => {
     )
   }
 
+  // 139's cost is pulled from Schedule 3 and stays served even mid-entry, because nothing on this
+  // page is an input to it; only 140's cost has a mirror. Extracted from a nested ternary (SonarQube
+  // 2026-08-21) -- worth doing, because the nesting hid that `derived` changes exactly ONE of these
+  // three branches.
+  const silvicultureCost = (code: number, item: LineItem | null): number | null | undefined => {
+    if (code === 139) return data.lessSilvAdminCost
+    if (code === 140) return derived ? derived.totalSilvicultureCost : data.totalSilvicultureCost
+    return item?.cost
+  }
+
+  // $/m³ = cost ÷ volume (139/140 fold in the Schedule 3 pulls). Mirrored while editable so it
+  // tracks entry; the document's server-computed figure otherwise (#291).
+  //
+  // NOTE the precedence is deliberately the OPPOSITE of `silvicultureCost` above: the mirror
+  // supersedes every row here, 139 and 140 included, because `deriveSchedule1` computes a rate for
+  // both (derived.ts:98,115) even though it computes a cost only for 140. Same two codes, different
+  // answer -- which is precisely what a three-deep ternary is bad at showing.
+  const silviculturePerUnit = (code: number, item: LineItem | null): number | null | undefined => {
+    if (derived) return derived.perUnit[code]
+    if (code === 139) return data.lessSilvAdminPerUnit
+    if (code === 140) return data.totalSilviculturePerUnit
+    return item?.perUnit
+  }
+
   const silvicultureRow = (row: (typeof SILV_ROWS)[number]) => {
     const item = data.silviculture[row.key]
     // All four silviculture VOLUMES are user-entered; only 1 & 2 have an editable cost. 139's cost is
     // pulled from Schedule 3, 140's is derived — both read-only.
     const writableCost = row.code === 1 || row.code === 2
-    // 139's cost is pulled from Schedule 3; 140's is the derived Total Silviculture cost (both read-only).
-    const costValue =
-      row.code === 139
-        ? data.lessSilvAdminCost
-        : row.code === 140
-          ? derived
-            ? derived.totalSilvicultureCost
-            : data.totalSilvicultureCost
-          : item?.cost
-    // $/m³ = cost ÷ volume (139/140 fold in the Schedule 3 pulls). Mirrored while editable so it
-    // tracks entry; the document's server-computed figure otherwise (#291).
-    const perUnitValue = derived
-      ? derived.perUnit[row.code]
-      : row.code === 139
-        ? data.lessSilvAdminPerUnit
-        : row.code === 140
-          ? data.totalSilviculturePerUnit
-          : item?.perUnit
+    const costValue = silvicultureCost(row.code, item)
+    const perUnitValue = silviculturePerUnit(row.code, item)
     return (
       <TableRow key={row.code}>
         <TableCell>{row.label}</TableCell>

frontend/src/components/schedule5SubPage/derived.ts

@@ -66,9 +66,11 @@ export function rowCostPerVolume(
 export function deriveSubPageTotals(
   kind: SubPageKind,
   rows: readonly SubPageRowForm[],
-  stampedVolume: number | null | undefined,
+  // Defaulted rather than normalised in the body (SonarQube 2026-08-21). Equivalent: a default fires
+  // only on `undefined`, and an explicitly-passed `null` already IS the value the body wants — so
+  // both the omitted and the null call paths land on `null`, as `?? null` did.
+  volume: number | null = null,
 ): SubPageTotals {
-  const volume = stampedVolume ?? null
   let cost = 0
   let contributed = false
   for (const row of rows) {