fix(schedule10): mirror the ballast D branch and disable the N-discarded inputs

Addresses both "worth fixing in this PR" items from SScholefield's review of #325.

1. Ballast method D had the same echo mismatch N was fixed for
-------------------------------------------------------------
buildStabilizing mirrored the server coercion only on N, so on D the request
carried whatever material had been picked BEFORE D was chosen -- the combo is
disabled on D, but the form still held the old value -- and
Schedule10Service:557-560 then silently replaced it with NA. That is precisely
the mismatch the N mirroring exists to prevent, reached by the other branch.

The material decision now goes through ballastForcesMaterialNa, which already
covered both branches, and is made independently of the figure zeroing. Only the
FIGURES are N-specific: D stores its figures as submitted, and still does.

2. Method N left the discarded inputs enabled
---------------------------------------------
The four dimensions plus actual cost and other transfer stayed editable while
buildStabilizing sent 0 for them, even though the material combo was disabled in
exactly the same situation. They are now disabled too, so the rule reads the same
way everywhere.

stTtTransfer is deliberately NOT in that set -- the server keeps it on the N
branch -- and the hint now names the affected fields and says so, rather than
leaving the reporter to work out which figures "dimensions, actual cost and other
transfer" meant.

The set lives in validation.ts as BALLAST_ZEROED_FIELDS, beside the builder that
zeroes them, so the disable list and the request body cannot drift apart.

Tests
-----
Three added, and all three were mutation-tested -- each fix reverted, the new
test confirmed failing, then restored:

- D sends NA for the material while every figure survives (this one fails with
  the old ballastZeroesFigures gate).
- N disables the six discarded inputs and leaves TtT Transfer editable.
- D disables only the material, leaving the figures editable.

Plus a drift guard: the fields buildStabilizing actually zeroes on N are compared
against BALLAST_ZEROED_FIELDS, so the disable list cannot silently diverge from
the request in either direction.

Verified: 202 tests green across Schedule 10 and every file this branch touches;
full frontend suite 1045/1046 with the one failure PRE-EXISTING (Schedule8
Helicopter, passes in isolation); build clean; lint and Prettier clean.

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

Author: Rylan-cgi

frontend/src/components/schedule10/RoadDetailFields.tsx

@@ -7,6 +7,7 @@ import CommaNumberInput from '@/components/core/CommaNumberInput'
 import { fmtCurrency, fmtWholeCost } from '@/utils/number'
 import type { MaskedField, RoadDetailErrors, RoadDetailFormValues } from './validation'
 import {
+  BALLAST_ZEROED_FIELDS,
   COMMENTS_MAX,
   ROAD_NAME_MAX,
   ballastForcesMaterialNa,
@@ -72,6 +73,18 @@ const RoadDetailFields: FC<RoadDetailFieldsProps> = ({
 }) => {
   const id = (name: string) => `${idPrefix}-${name}`
 
+  // `N` and `D` both have their material forced to `NA`; `N` additionally has its dimensions and two
+  // of its three costs zeroed, which `buildStabilizing` sends rather than leaving to the server.
+  const materialForced = ballastForcesMaterialNa(form.stBallastMethodCode)
+  const figuresZeroed = ballastZeroesFigures(form.stBallastMethodCode)
+
+  // An input whose entry `N` discards is disabled, so the rule reads the same way everywhere: the
+  // material combo was already disabled in exactly this situation, and leaving these editable
+  // invited entry that Save would silently replace with zero. `stTtTransfer` is NOT in the set —
+  // the server keeps it on the `N` branch, so it stays editable.
+  const zeroedByBallast = (key: MaskedField): boolean =>
+    figuresZeroed && (BALLAST_ZEROED_FIELDS as readonly string[]).includes(key)
+
   // A stored classification may have been de-listed since it was saved, in which case it is absent
   // from the offerable list. Appending it keeps the field showing what the row actually holds instead
   // of appearing unselected — and it is appended with the row's OWN label, which the response carries
@@ -130,7 +143,7 @@ const RoadDetailFields: FC<RoadDetailFieldsProps> = ({
           labelText={labelWithUnit}
           autoComplete="off"
           value={form[key]}
-          disabled={disabled}
+          disabled={disabled || zeroedByBallast(key)}
           invalid={Boolean(errors[key])}
           invalidText={errors[key] ?? ''}
           onValueChange={(raw) => onChange(key, raw)}
@@ -167,11 +180,6 @@ const RoadDetailFields: FC<RoadDetailFieldsProps> = ({
     )
   }
 
-  // `N` and `D` both have their material forced to `NA`; `N` additionally has its dimensions and two
-  // of its three costs zeroed, which `buildStabilizing` now sends rather than leaving to the server.
-  const materialForced = ballastForcesMaterialNa(form.stBallastMethodCode)
-  const figuresZeroed = ballastZeroesFigures(form.stBallastMethodCode)
-
   const engineeringCostsValue = form.detailedEngineeringCostInd === 'Y' ? 'Yes' : 'No'
 
   const endHaulRate = previewCostPerVolumePerLength(
@@ -315,8 +323,9 @@ const RoadDetailFields: FC<RoadDetailFieldsProps> = ({
         )}
       {!readOnly && figuresZeroed && (
         <p className="schedule-10__hint">
-          This Additional Stabilizing code stores its dimensions, actual cost and other transfer as
-          zero.
+          This Additional Stabilizing code stores its length, surface width, depth, distance to
+          source, actual cost and other transfer as zero, so those fields are disabled. TtT Transfer
+          is still recorded as entered.
         </p>
       )}
     </div>

frontend/src/components/schedule10/__tests__/Schedule10.test.tsx

@@ -1307,6 +1307,59 @@ describe('regressions from the 2026-08-19 code review', () => {
     })
   })
 
+  test('review #325 — method N disables the figures it discards, but not TtT Transfer', async () => {
+    renderSchedule10('/schedule-10?pageId=8900')
+    await userEvent.click(await screen.findByRole('button', { name: 'Edit' }))
+    await screen.findByDisplayValue('Mainline A')
+
+    // Before choosing N every figure is editable.
+    expect(screen.getByLabelText('Additional Stabilizing Length (km)')).toBeEnabled()
+
+    await userEvent.click(screen.getByRole('combobox', { name: 'Ballast Method Code' }))
+    await userEvent.click(await screen.findByRole('option', { name: 'None' }))
+
+    for (const label of [
+      'Additional Stabilizing Length (km)',
+      'Additional Stabilizing Surface Width (m)',
+      'Depth (m)',
+      'Distance to Source (km)',
+      'Additional Stabilizing Actual Costs ($)',
+      'Additional Stabilizing Other Transfer ($)',
+    ]) {
+      expect(screen.getByLabelText(label)).toBeDisabled()
+    }
+    // The server keeps this one on the N branch, so entry here is still recorded.
+    expect(screen.getByLabelText('Additional Stabilizing TtT Transfer ($)')).toBeEnabled()
+  })
+
+  test('review #325 — method D disables only the material, leaving the figures editable', async () => {
+    server.use(
+      getHandler(
+        doc({
+          codeLists: {
+            ...doc().codeLists,
+            ballastMethods: [
+              { code: 'C', description: 'Crushed' },
+              { code: 'N', description: 'None' },
+              { code: 'D', description: 'Dirt' },
+            ],
+          },
+        }),
+      ),
+    )
+    renderSchedule10('/schedule-10?pageId=8900')
+    await userEvent.click(await screen.findByRole('button', { name: 'Edit' }))
+    await screen.findByDisplayValue('Mainline A')
+
+    await userEvent.click(screen.getByRole('combobox', { name: 'Ballast Method Code' }))
+    await userEvent.click(await screen.findByRole('option', { name: 'Dirt' }))
+
+    expect(screen.getByRole('combobox', { name: 'Type' })).toBeDisabled()
+    // D stores its figures as submitted, so they must stay editable.
+    expect(screen.getByLabelText('Additional Stabilizing Length (km)')).toBeEnabled()
+    expect(screen.getByLabelText('Additional Stabilizing Actual Costs ($)')).toBeEnabled()
+  })
+
   test('M7 — a de-listed BEC classification renders its label, not its catalogue id', async () => {
     server.use(
       getHandler(

frontend/src/components/schedule10/__tests__/validation.test.ts

@@ -1,6 +1,7 @@
 import { describe, expect, test } from 'vitest'
 import type { ConstructionPage, RoadDetail } from '@/interfaces/Schedule10Response'
 import {
+  BALLAST_ZEROED_FIELDS,
   MASK_DIGITS,
   SCH10_MESSAGES,
   ballastForcesMaterialNa,
@@ -722,6 +723,69 @@ describe('fixes from the 2026-08-19 code review', () => {
     })
   })
 
+  test('review #325 — method D forces the material to NA but keeps every figure', () => {
+    // The N mirroring originally covered only N, so D sent whatever material was picked before D was
+    // chosen and the server (Schedule10Service:557-560) silently replaced it with NA -- the exact
+    // echo mismatch the mirroring exists to prevent. D coerces the material ONLY; its figures are
+    // stored as submitted.
+    const form = {
+      ...base(),
+      stBallastMethodCode: 'D',
+      stBallastMaterialCode: 'GR',
+      stLength: '3',
+      stSurfaceWidth: '6.5',
+      stDepth: '0.3',
+      stDistanceToSource: '12.4',
+      stActualCost: '5000',
+      stTtTransfer: '750',
+      stOtherTransfer: '250',
+    }
+    expect(buildRoadDetailBody(form).stabilizing).toEqual({
+      ballastMethodCode: 'D',
+      ballastMaterialCode: 'NA',
+      length: 3,
+      surfaceWidth: 6.5,
+      depth: 0.3,
+      distanceToSource: 12.4,
+      actualCost: 5000,
+      ttTransfer: 750,
+      otherTransfer: 250,
+    })
+  })
+
+  test('review #325 — the zeroed-field set matches what the N branch actually sends', () => {
+    // BALLAST_ZEROED_FIELDS drives which inputs the form DISABLES. If it drifts from what
+    // buildStabilizing zeroes, the UI either disables a field that is still recorded or leaves one
+    // editable whose entry is discarded -- the bug this review caught, in the other direction.
+    const filled = {
+      ...base(),
+      stBallastMethodCode: 'N',
+      stLength: '3',
+      stSurfaceWidth: '6.5',
+      stDepth: '0.3',
+      stDistanceToSource: '12.4',
+      stActualCost: '5000',
+      stTtTransfer: '750',
+      stOtherTransfer: '250',
+    }
+    const sent = buildRoadDetailBody(filled).stabilizing as Record<string, unknown>
+    // Map each form key to the request field it lands in.
+    const requestField: Record<string, string> = {
+      stLength: 'length',
+      stSurfaceWidth: 'surfaceWidth',
+      stDepth: 'depth',
+      stDistanceToSource: 'distanceToSource',
+      stActualCost: 'actualCost',
+      stOtherTransfer: 'otherTransfer',
+      stTtTransfer: 'ttTransfer',
+    }
+    const zeroed = Object.keys(requestField).filter((key) => sent[requestField[key]] === 0)
+    expect(zeroed.sort()).toEqual([...BALLAST_ZEROED_FIELDS].sort())
+    // And the one deliberately left out is genuinely still carried.
+    expect(BALLAST_ZEROED_FIELDS).not.toContain('stTtTransfer')
+    expect(sent.ttTransfer).toBe(750)
+  })
+
   test('P2 — both N and D force the material to NA in the UI', () => {
     expect(ballastForcesMaterialNa('N')).toBe(true)
     expect(ballastForcesMaterialNa('D')).toBe(true)

frontend/src/components/schedule10/validation.ts

@@ -188,6 +188,20 @@ export const ballastMaterialRequired = (methodCode: string): boolean => {
   return code === '' || code === 'C'
 }
 
+/**
+ * The Additional Stabilizing figures ballast method `N` stores as zero, so the form can disable
+ * exactly the inputs whose entry would be discarded. `stTtTransfer` is deliberately ABSENT: the
+ * server keeps it on the `N` branch, so it stays editable.
+ */
+export const BALLAST_ZEROED_FIELDS = [
+  'stLength',
+  'stSurfaceWidth',
+  'stDepth',
+  'stDistanceToSource',
+  'stActualCost',
+  'stOtherTransfer',
+] as const
+
 /** The material code the server substitutes on the `N` and `D` branches. */
 export const BALLAST_MATERIAL_NA = 'NA'
 
@@ -197,7 +211,11 @@ export const ballastForcesMaterialNa = (methodCode: string): boolean => {
   return code === 'N' || code === 'D'
 }
 
-/** Ballast method `N` is the branch whose dimensions and two costs the server forces to zero. */
+/**
+ * Ballast method `N` is the branch whose dimensions and two costs the server forces to zero. The
+ * MATERIAL is not this branch's business — `D` forces that too, so it goes through
+ * {@link ballastForcesMaterialNa}.
+ */
 export const ballastZeroesFigures = (methodCode: string): boolean =>
   methodCode.trim().toUpperCase() === 'N'
 
@@ -580,10 +598,19 @@ const buildSubGrade = (form: RoadDetailFormValues): SubGradeRequest => ({
  */
 const buildStabilizing = (form: RoadDetailFormValues): StabilizingRequest => {
   const method = form.stBallastMethodCode.trim()
+  // BOTH `N` and `D` have the material forced to `NA` server-side
+  // (`Schedule10Service.java:557-560`), so both must send it. Mirroring only `N` left `D` with the
+  // exact echo mismatch the mirroring exists to prevent: the combo is disabled on `D`, but the form
+  // still held whatever material was picked BEFORE `D` was chosen, and that value was sent and then
+  // silently replaced. Only the FIGURES are `N`-specific.
+  const ballastMaterialCode = ballastForcesMaterialNa(method)
+    ? BALLAST_MATERIAL_NA
+    : blankToNull(form.stBallastMaterialCode)
+
   if (ballastZeroesFigures(method)) {
     return {
       ballastMethodCode: method,
-      ballastMaterialCode: BALLAST_MATERIAL_NA,
+      ballastMaterialCode,
       length: 0,
       surfaceWidth: 0,
       depth: 0,
@@ -596,7 +623,7 @@ const buildStabilizing = (form: RoadDetailFormValues): StabilizingRequest => {
   }
   return {
     ballastMethodCode: method,
-    ballastMaterialCode: blankToNull(form.stBallastMaterialCode),
+    ballastMaterialCode,
     length: numberOrNull(form.stLength),
     surfaceWidth: numberOrNull(form.stSurfaceWidth),
     depth: numberOrNull(form.stDepth),