refactor(categories): migration target picker lists leaves only

The v1 target combobox in the migration preview now offers only leaf
categories (the inputable end-categories), not intermediate parents, so a
transaction is never mapped to a grouping bucket. Leaves render as a flat,
un-indented list in taxonomy order.

A low-confidence default can still point a v2 category at a non-leaf parent
(e.g. 'Divertissement' #1710); MappingRow re-injects that current target for
its single row via findTaxonomyCategory so the input stays populated, while
new picks remain leaves-only. Adds a findTaxonomyCategory pure helper + tests
(adapter now returns 112 leaves, not the 150-node full tree).

src/components/categories-migration/MappingRow.tsx

@@ -1,3 +1,4 @@
+import { useMemo } from "react";
 import { useTranslation } from "react-i18next";
 import CategoryCombobox from "../shared/CategoryCombobox";
 import type { Category } from "../../shared/types";
@@ -22,10 +23,18 @@ interface MappingRowProps {
   /** Number of transactions currently attached to this v2 category. */
   transactionCount: number;
   /**
-   * v1 leaf catalogue adapted to the `Category` shape the combobox expects.
-   * Computed ONCE by StepSimulate and shared across rows (perf).
+   * v1 LEAF catalogue adapted to the `Category` shape the combobox expects.
+   * Computed ONCE by StepSimulate and shared across rows (perf). Leaves only —
+   * a transaction is never mapped to a grouping bucket.
    */
   targetCategories: Category[];
+  /**
+   * Resolves a target id (possibly a non-leaf parent absent from the leaves-only
+   * `targetCategories`) to a display `Category`. Used to keep a low-confidence
+   * parent default like "Divertissement" (#1710) visible on its row without
+   * offering parents as new picks.
+   */
+  resolveTarget: (id: number) => Category | null;
 }
 
 function badgeClass(confidence: ConfidenceBadge): string {
@@ -48,9 +57,23 @@ export default function MappingRow({
   onResolve,
   transactionCount,
   targetCategories,
+  resolveTarget,
 }: MappingRowProps) {
   const { t } = useTranslation();
 
+  // The picker lists leaves only. If this row's current target is a non-leaf
+  // parent (a low-confidence default like "Divertissement" #1710), it is absent
+  // from that list and would blank the combobox input, so re-inject it for this
+  // row only — shown as the current value, never offered as a new pick.
+  const comboboxCategories = useMemo(() => {
+    const id = row.v1TargetId;
+    if (id == null || targetCategories.some((c) => c.id === id)) {
+      return targetCategories;
+    }
+    const current = resolveTarget(id);
+    return current ? [...targetCategories, current] : targetCategories;
+  }, [targetCategories, row.v1TargetId, resolveTarget]);
+
   const badgeLabel = t(
     `categoriesSeed.migration.simulate.confidence.${row.confidence}`,
   );
@@ -136,7 +159,7 @@ export default function MappingRow({
       >
         <div className="w-full max-w-[16rem]">
           <CategoryCombobox
-            categories={targetCategories}
+            categories={comboboxCategories}
             value={row.v1TargetId ?? null}
             onChange={handleTargetChange}
             compact

src/components/categories-migration/StepSimulate.tsx

@@ -1,9 +1,12 @@
-import { useMemo } from "react";
+import { useMemo, useCallback } from "react";
 import { useTranslation } from "react-i18next";
 import { ArrowLeft, ArrowRight, AlertTriangle, FolderHeart } from "lucide-react";
 import MappingRow from "./MappingRow";
 import TransactionPreviewPanel from "./TransactionPreviewPanel";
-import { taxonomyToComboboxCategories } from "./migrationTargets";
+import {
+  taxonomyToComboboxCategories,
+  findTaxonomyCategory,
+} from "./migrationTargets";
 import { useCategoryTaxonomy } from "../../hooks/useCategoryTaxonomy";
 import type {
   MigrationPlan,
@@ -40,15 +43,23 @@ export default function StepSimulate({
   const { t } = useTranslation();
   const { taxonomy } = useCategoryTaxonomy();
 
-  // Full v1 taxonomy adapted to the combobox `Category` shape (hierarchical).
-  // Computed once here (not per row) and shared across every MappingRow's
-  // target picker. Includes non-leaf parents so mid-tree default targets like
-  // "Divertissement" (id 1710) display and can be re-picked.
+  // v1 LEAF catalogue adapted to the combobox `Category` shape. Computed once
+  // here (not per row) and shared across every MappingRow's target picker —
+  // only leaves are offered, so a transaction is never mapped to a grouping
+  // bucket.
   const targetCategories = useMemo(
     () => taxonomyToComboboxCategories(taxonomy.roots),
     [taxonomy],
   );
 
+  // Resolve a target id (possibly a non-leaf parent absent from the leaves-only
+  // list) to a display Category, so a row keeps showing a low-confidence parent
+  // default like "Divertissement" (#1710) without offering parents as picks.
+  const resolveTarget = useCallback(
+    (id: number) => findTaxonomyCategory(taxonomy.roots, id),
+    [taxonomy],
+  );
+
   const selectedRow = useMemo<MappingRowType | null>(() => {
     if (selectedRowV2Id === null) return null;
     return (
@@ -162,6 +173,7 @@ export default function StepSimulate({
                   transactionCountByV2Id.get(row.v2CategoryId) ?? 0
                 }
                 targetCategories={targetCategories}
+                resolveTarget={resolveTarget}
               />
             </li>
           ))}

src/components/categories-migration/migrationTargets.test.ts

@@ -1,5 +1,8 @@
 import { describe, it, expect } from "vitest";
-import { taxonomyToComboboxCategories } from "./migrationTargets";
+import {
+  taxonomyToComboboxCategories,
+  findTaxonomyCategory,
+} from "./migrationTargets";
 import {
   getTaxonomyV1,
   type TaxonomyNode,
@@ -22,38 +25,50 @@ function node(
   };
 }
 
-describe("taxonomyToComboboxCategories", () => {
+describe("taxonomyToComboboxCategories (leaves only)", () => {
   it("returns [] for empty input", () => {
     expect(taxonomyToComboboxCategories([])).toEqual([]);
   });
 
-  it("flattens the tree in parent-before-children (DFS) order", () => {
+  it("keeps only leaves, dropping every intermediate parent, in DFS order", () => {
     const roots = [
-      node(1000, "Revenus", [node(1010, "Emploi", [node(1011, "Paie")])]),
+      node(1000, "Revenus", [
+        node(1010, "Emploi", [node(1011, "Paie"), node(1012, "Prime")]),
+      ]),
       node(1100, "Alimentation", [node(1111, "Épicerie")]),
     ];
     const out = taxonomyToComboboxCategories(roots);
-    expect(out.map((c) => c.id)).toEqual([1000, 1010, 1011, 1100, 1111]);
+    // parents 1000/1010/1100 dropped; leaves kept in depth-first reading order
+    expect(out.map((c) => c.id)).toEqual([1011, 1012, 1111]);
   });
 
-  it("wires real parent_id (undefined for roots) so the combobox can nest", () => {
-    const roots = [node(1000, "Revenus", [node(1010, "Emploi", [node(1011, "Paie")])])];
-    const byId = new Map(taxonomyToComboboxCategories(roots).map((c) => [c.id, c]));
-    expect(byId.get(1000)!.parent_id).toBeUndefined();
-    expect(byId.get(1010)!.parent_id).toBe(1000);
-    expect(byId.get(1011)!.parent_id).toBe(1010);
+  it("flattens leaves: parent_id undefined + running sort_order (flat list)", () => {
+    const roots = [
+      node(1000, "Revenus", [
+        node(1010, "Emploi", [node(1011, "Paie"), node(1012, "Prime")]),
+      ]),
+      node(1100, "Alimentation", [node(1111, "Épicerie")]),
+    ];
+    const out = taxonomyToComboboxCategories(roots);
+    expect(out.every((c) => c.parent_id === undefined)).toBe(true);
+    expect(out.map((c) => c.sort_order)).toEqual([0, 1, 2]);
   });
 
-  it("marks only leaves inputable; parents are selectable but non-inputable", () => {
-    const roots = [node(1700, "Loisirs", [node(1710, "Divertissement", [node(1711, "Cinéma")])])];
-    const byId = new Map(taxonomyToComboboxCategories(roots).map((c) => [c.id, c]));
-    expect(byId.get(1700)!.is_inputable).toBe(false);
-    expect(byId.get(1710)!.is_inputable).toBe(false); // intermediate parent
-    expect(byId.get(1711)!.is_inputable).toBe(true); // leaf
+  it("marks every returned row inputable (all leaves)", () => {
+    const roots = [
+      node(1700, "Loisirs", [
+        node(1710, "Divertissement", [node(1711, "Cinéma")]),
+      ]),
+    ];
+    const out = taxonomyToComboboxCategories(roots);
+    expect(out.map((c) => c.id)).toEqual([1711]); // 1700 + 1710 parents dropped
+    expect(out.every((c) => c.is_inputable)).toBe(true);
   });
 
   it("copies id/name/type/color/i18n_key and defaults is_active + created_at", () => {
-    const [c] = taxonomyToComboboxCategories([node(1111, "Épicerie", [], "expense")]);
+    const [c] = taxonomyToComboboxCategories([
+      node(1111, "Épicerie", [], "expense"),
+    ]);
     expect(c).toMatchObject({
       id: 1111,
       name: "Épicerie",
@@ -62,22 +77,53 @@ describe("taxonomyToComboboxCategories", () => {
       i18n_key: "categoriesSeed.test.1111",
       is_active: true,
       is_inputable: true,
-      sort_order: 1,
     });
     expect(typeof c.created_at).toBe("string");
   });
 
-  it("adapts the real bundled v1 taxonomy: every node present, ids unique, non-leaf target 1710 offered", () => {
+  it("adapts the real v1 taxonomy: 112 leaves, ids unique, NO non-leaf parent offered", () => {
     const out = taxonomyToComboboxCategories(getTaxonomyV1().roots);
-    expect(out.length).toBe(150); // 11 roots + 27 intermediates + 112 leaves
+    expect(out.length).toBe(112); // leaves only (was 150 incl. 38 parents)
     const ids = new Set(out.map((c) => c.id));
     expect(ids.size).toBe(out.length);
-    // The migration default maps v2 cat 26 → v1 1710 (a non-leaf parent), so it
-    // must be selectable/displayable in the picker.
-    const divertissement = out.find((c) => c.id === 1710);
-    expect(divertissement).toBeDefined();
-    expect(divertissement!.is_inputable).toBe(false);
-    // Leaves stay inputable — a random known leaf.
+    // The non-leaf parent 1710 "Divertissement" must NOT be a selectable option.
+    expect(out.find((c) => c.id === 1710)).toBeUndefined();
+    // A known leaf is present and inputable.
     expect(out.find((c) => c.id === 1111)!.is_inputable).toBe(true);
+    expect(out.every((c) => c.is_inputable)).toBe(true);
+  });
+});
+
+describe("findTaxonomyCategory", () => {
+  const roots = [
+    node(1700, "Loisirs", [
+      node(1710, "Divertissement", [node(1711, "Cinéma")]),
+    ]),
+  ];
+
+  it("resolves a non-leaf parent by id (to display a current target)", () => {
+    const c = findTaxonomyCategory(roots, 1710);
+    expect(c).toMatchObject({
+      id: 1710,
+      name: "Divertissement",
+      is_inputable: false,
+    });
+    expect(c!.parent_id).toBeUndefined();
+  });
+
+  it("resolves a leaf too", () => {
+    expect(findTaxonomyCategory(roots, 1711)).toMatchObject({
+      id: 1711,
+      is_inputable: true,
+    });
+  });
+
+  it("returns null for an unknown id", () => {
+    expect(findTaxonomyCategory(roots, 9999)).toBeNull();
+  });
+
+  it("finds the real low-confidence default target 1710 in the bundled taxonomy", () => {
+    const c = findTaxonomyCategory(getTaxonomyV1().roots, 1710);
+    expect(c).toMatchObject({ id: 1710, is_inputable: false });
   });
 });

src/components/categories-migration/migrationTargets.ts

@@ -2,42 +2,80 @@ import type { Category } from "../../shared/types";
 import type { TaxonomyNode } from "../../services/categoryTaxonomyService";
 
 /**
- * Flatten the v1 taxonomy tree into `Category`-shaped rows that
- * `CategoryCombobox` can render (hierarchical, accent-insensitive type-ahead).
+ * Flatten the v1 taxonomy into `Category`-shaped rows for the migration target
+ * picker, keeping ONLY the leaves — the inputable end-categories a transaction
+ * can actually be filed under. Intermediate parents (grouping buckets) are
+ * dropped so the type-ahead offers real targets only, never a bucket.
  *
- * We map EVERY node — roots, intermediate parents AND leaves — not only the
- * leaves: the migration's default mapping table can point a v2 category at a
- * non-leaf parent (e.g. v2 "Jeux, Films & Livres" → v1 "Divertissement",
- * id 1710), so the picker must be able to both display and offer those nodes.
- * Feeding the full tree also lets the combobox reproduce the taxonomy hierarchy
- * (parents before children, siblings by sort order) — the same shape the
- * by-category report picker already uses.
+ * Leaves are emitted in depth-first reading order and carry a running
+ * `sort_order` with `parent_id` undefined, so `CategoryCombobox` renders them as
+ * one flat, un-indented list in taxonomy order (leaves stay grouped under their
+ * original parent) instead of re-scrambling them by the per-parent sort_order.
  *
- * Each row carries its real `parent_id` and `sort_order`; `is_inputable`
- * mirrors the DB (leaves only), matching what the migration writer inserts via
- * `listAllV1Rows()`. Any id the user can pick here therefore exists in the DB
- * after the migration, so the resulting mapping is always FK-safe.
+ * Every leaf id is `is_inputable` in the DB after the migration, so any pick is
+ * FK-safe.
  *
- * Pure helper — no i18n, no DB, no React. The display name is resolved by the
- * combobox at render time via `i18n_key` (falling back to `name`).
+ * Edge case: a low-confidence default can point a v2 category at a NON-leaf
+ * parent (e.g. v2 "Jeux, Films & Livres" -> v1 "Divertissement" #1710). Such a
+ * value is absent from this leaves-only list; `MappingRow` re-injects it for that
+ * single row (via `findTaxonomyCategory`) so the input still shows the current
+ * suggestion, while new picks stay leaves-only.
+ *
+ * Pure helper — no i18n, no DB, no React.
  */
 export function taxonomyToComboboxCategories(roots: TaxonomyNode[]): Category[] {
   const out: Category[] = [];
-  const walk = (node: TaxonomyNode, parentId: number | null): void => {
-    out.push({
-      id: node.id,
-      name: node.name,
-      parent_id: parentId ?? undefined,
-      color: node.color,
-      type: node.type,
-      is_active: true,
-      is_inputable: node.children.length === 0,
-      sort_order: node.sort_order,
-      i18n_key: node.i18n_key,
-      created_at: "",
-    });
-    for (const child of node.children) walk(child, node.id);
+  let order = 0;
+  const walk = (node: TaxonomyNode): void => {
+    if (node.children.length === 0) {
+      out.push({
+        id: node.id,
+        name: node.name,
+        parent_id: undefined,
+        color: node.color,
+        type: node.type,
+        is_active: true,
+        is_inputable: true,
+        sort_order: order++,
+        i18n_key: node.i18n_key,
+        created_at: "",
+      });
+      return;
+    }
+    for (const child of node.children) walk(child);
   };
-  for (const root of roots) walk(root, null);
+  for (const root of roots) walk(root);
   return out;
 }
+
+/**
+ * Resolve any taxonomy node (leaf OR parent) by id into the `Category` shape,
+ * for DISPLAY of a current non-leaf target that the leaves-only picker omits.
+ * `sort_order` is pushed to the end so, when injected, the node sorts last.
+ * Returns null when the id is absent from the taxonomy. Pure helper.
+ */
+export function findTaxonomyCategory(
+  roots: TaxonomyNode[],
+  id: number,
+): Category | null {
+  const stack: TaxonomyNode[] = [...roots];
+  while (stack.length > 0) {
+    const node = stack.pop()!;
+    if (node.id === id) {
+      return {
+        id: node.id,
+        name: node.name,
+        parent_id: undefined,
+        color: node.color,
+        type: node.type,
+        is_active: true,
+        is_inputable: node.children.length === 0,
+        sort_order: Number.MAX_SAFE_INTEGER,
+        i18n_key: node.i18n_key,
+        created_at: "",
+      };
+    }
+    for (const child of node.children) stack.push(child);
+  }
+  return null;
+}