From a8b2040216f718ec9ed752e2a100acc81de2015c Mon Sep 17 00:00:00 2001
From: Bryan Van Deusen <bryan.vandeusen@fabledsword.com>
Date: Sun, 30 Aug 2026 12:52:07 -0400
Subject: [PATCH] feat(rules): the edit history is visible in the slide-over
 (#3243, milestone 323 step 4)
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Rule 27: a history nobody can read is not shipped. `RuleHistoryPanel.vue`
sits below the fields in `RuleEditorSlideOver`, where a rule is read in full
— not on the list row, where a history entry point would compete with the
row's job.

REUSE, DECIDED FIELD BY FIELD RATHER THAN ALL AT ONCE.

DiffView.vue is reused unchanged: it takes DiffLine[] and nothing
note-shaped.

HistoryPanel.vue is NOT, and its props are the reason — noteId +
currentBody, a NoteVersion carrying tags and pin columns, a fetch of
/api/notes/…, a restore emit, pin/unpin buttons. Rules have no tags, no
pins, and deliberately no restore, and a rule's text is EIGHT fields rather
than one body, which changes the reader's question from "what changed" to
"which fields moved". Recorded here rather than forked silently, per #3207.

THE FORK THAT WAS ALREADY THERE. The LCS walk existed three times —
privately in useAssist.ts, and again inside HistoryPanel.vue and
VersionHistorySection.vue — character-identical apart from quote style,
because computeDiff was never exported. Rather than add a fourth copy, it
moves to utils/diff.ts and the three become imports; the extraction was
verified equivalent to all three before anything was deleted. DiffLine is
re-exported from useAssist so its existing importers are untouched.

WHAT A ROW SHOWS: when, and which fields moved. A version holds the text the
edit REPLACED, so the edit is the step from a row to the next NEWER state —
the row above it, or, for the newest row, the rule as it stands now.
Comparing against the row below would attribute every change to the wrong
edit. A field nobody has fetched yet reads as neither changed nor unchanged.

An edit that touched verify_with is badged "check reset", because that edit
silently cleared verified_at (milestone 312) and put the rule back at the
top of the staleness sweep — a moment visible nowhere else.

The badge is a 12% color-mix TINT, not solid `--fs-warning`.
`--fs-warning-fg` is defined in theme.css as "warning TEXT on a warning
tint", so painting it over the solid token is exactly the same-hue contrast
failure #3141 records. Every var() the component references resolves against
theme.css, checked before pushing.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
---
 frontend/src/api/rulebooks.ts                 |  43 +++
 frontend/src/components/HistoryPanel.vue      |  24 +-
 .../src/components/VersionHistorySection.vue  |  22 +-
 .../components/rules/RuleEditorSlideOver.vue  |  12 +
 .../src/components/rules/RuleHistoryPanel.vue | 274 ++++++++++++++++++
 frontend/src/composables/useAssist.ts         |  31 +-
 frontend/src/utils/diff.ts                    |  50 ++++
 7 files changed, 388 insertions(+), 68 deletions(-)
 create mode 100644 frontend/src/components/rules/RuleHistoryPanel.vue
 create mode 100644 frontend/src/utils/diff.ts

diff --git a/frontend/src/api/rulebooks.ts b/frontend/src/api/rulebooks.ts
index f884999..9665f42 100644
--- a/frontend/src/api/rulebooks.ts
+++ b/frontend/src/api/rulebooks.ts
@@ -228,6 +228,49 @@ export async function unrelateRules(relationId: number): Promise<void> {
   return apiDelete(`/api/rule-relations/${relationId}`);
 }
 
+/**
+ * One entry in a rule's edit history.
+ *
+ * Each entry holds the text the edit REPLACED, not the text it introduced —
+ * so the newest entry is what the rule said before its most recent change,
+ * and what that change produced is the rule as it stands now. Read the other
+ * way round, every diff comes out backwards.
+ *
+ * The listing form omits the long fields; open one to get them.
+ */
+export interface RuleVersion {
+  id: number;
+  rule_id: number;
+  /** Who made the edit. Null when that account has since been deleted. */
+  user_id: number | null;
+  title: string;
+  created_at: string;
+  statement?: string;
+  why?: string;
+  how_to_apply?: string;
+  when_to_apply?: string;
+  tier?: string;
+  verify_with?: string;
+  expires_when?: string;
+}
+
+export async function listRuleVersions(ruleId: number): Promise<RuleVersion[]> {
+  const data = await apiGet<{ versions: RuleVersion[] }>(
+    `/api/rules/${ruleId}/versions`,
+  );
+  return data.versions;
+}
+
+export async function getRuleVersion(
+  ruleId: number, versionId: number,
+): Promise<RuleVersion> {
+  return apiGet<RuleVersion>(`/api/rules/${ruleId}/versions/${versionId}`);
+}
+
+// No restoreRuleVersion, deliberately (milestone 323). Putting an old wording
+// back goes through updateRule, which snapshots what it replaces — so the
+// undo stays visible in the history like any other edit.
+
 export async function deleteRule(id: number): Promise<void> {
   return apiDelete(`/api/rules/${id}`);
 }
diff --git a/frontend/src/components/HistoryPanel.vue b/frontend/src/components/HistoryPanel.vue
index 0688a9c..56491bc 100644
--- a/frontend/src/components/HistoryPanel.vue
+++ b/frontend/src/components/HistoryPanel.vue
@@ -2,7 +2,7 @@
 import { ref, computed, onMounted } from "vue";
 import { apiGet, pinNoteVersion, unpinNoteVersion } from "@/api/client";
 import DiffView from "@/components/DiffView.vue";
-import type { DiffLine } from "@/composables/useAssist";
+import { computeDiff, type DiffLine } from "@/utils/diff";
 import { fmtStamp } from "@/utils/dateFormat";
 
 interface NoteVersion {
@@ -33,28 +33,8 @@ const loadingDetail = ref(false);
 
 const diff = computed<DiffLine[]>(() => {
   if (!selectedVersion.value?.body) return [];
-  const a = props.currentBody;
-  const b = selectedVersion.value.body;
 
-  const aLines = a.split('\n');
-  const bLines = b.split('\n');
-  const m = aLines.length, n = bLines.length;
-  const dp: number[][] = Array.from({ length: m + 1 }, () => new Array(n + 1).fill(0));
-  for (let i = m - 1; i >= 0; i--)
-    for (let j = n - 1; j >= 0; j--)
-      dp[i][j] = aLines[i] === bLines[j]
-        ? dp[i+1][j+1] + 1
-        : Math.max(dp[i+1][j], dp[i][j+1]);
-  const result: DiffLine[] = [];
-  let i = 0, j = 0;
-  while (i < m && j < n) {
-    if (aLines[i] === bLines[j]) { result.push({ type: 'equal', text: aLines[i++] }); j++; }
-    else if (dp[i+1][j] >= dp[i][j+1]) result.push({ type: 'delete', text: aLines[i++] });
-    else result.push({ type: 'insert', text: bLines[j++] });
-  }
-  while (i < m) result.push({ type: 'delete', text: aLines[i++] });
-  while (j < n) result.push({ type: 'insert', text: bLines[j++] });
-  return result;
+  return computeDiff(props.currentBody, selectedVersion.value.body);
 });
 
 async function loadVersions() {
diff --git a/frontend/src/components/VersionHistorySection.vue b/frontend/src/components/VersionHistorySection.vue
index 6c7cc16..fb344c6 100644
--- a/frontend/src/components/VersionHistorySection.vue
+++ b/frontend/src/components/VersionHistorySection.vue
@@ -2,7 +2,7 @@
 import { ref, computed } from "vue";
 import { apiGet } from "@/api/client";
 import DiffView from "@/components/DiffView.vue";
-import type { DiffLine } from "@/composables/useAssist";
+import { computeDiff, type DiffLine } from "@/utils/diff";
 
 interface NoteVersion {
   id: number;
@@ -31,25 +31,7 @@ const loadingDetail = ref(false);
 
 const diff = computed<DiffLine[]>(() => {
   if (!selectedVersion.value?.body) return [];
-  const aLines = props.currentBody.split("\n");
-  const bLines = selectedVersion.value.body.split("\n");
-  const m = aLines.length, n = bLines.length;
-  const dp: number[][] = Array.from({ length: m + 1 }, () => new Array(n + 1).fill(0));
-  for (let i = m - 1; i >= 0; i--)
-    for (let j = n - 1; j >= 0; j--)
-      dp[i][j] = aLines[i] === bLines[j]
-        ? dp[i + 1][j + 1] + 1
-        : Math.max(dp[i + 1][j], dp[i][j + 1]);
-  const result: DiffLine[] = [];
-  let i = 0, j = 0;
-  while (i < m && j < n) {
-    if (aLines[i] === bLines[j]) { result.push({ type: "equal", text: aLines[i++] }); j++; }
-    else if (dp[i + 1][j] >= dp[i][j + 1]) result.push({ type: "delete", text: aLines[i++] });
-    else result.push({ type: "insert", text: bLines[j++] });
-  }
-  while (i < m) result.push({ type: "delete", text: aLines[i++] });
-  while (j < n) result.push({ type: "insert", text: bLines[j++] });
-  return result;
+  return computeDiff(props.currentBody, selectedVersion.value.body);
 });
 
 function formatDate(iso: string): string {
diff --git a/frontend/src/components/rules/RuleEditorSlideOver.vue b/frontend/src/components/rules/RuleEditorSlideOver.vue
index 26523bf..22f54c4 100644
--- a/frontend/src/components/rules/RuleEditorSlideOver.vue
+++ b/frontend/src/components/rules/RuleEditorSlideOver.vue
@@ -3,6 +3,7 @@ import { computed, ref, watch, onMounted } from "vue";
 import { useRulebooksStore } from "@/stores/rulebooks";
 import { useCanonicalSystemsStore } from "@/stores/canonicalSystems";
 import type { RuleTier } from "@/api/rulebooks";
+import RuleHistoryPanel from "@/components/rules/RuleHistoryPanel.vue";
 
 const props = defineProps<{ ruleId: number | null; topicId: number | null }>();
 const emit = defineEmits<{ close: [] }>();
@@ -256,6 +257,17 @@ watch(() => props.ruleId, load);
         How to apply
         <textarea v-model="howToApply" rows="4" placeholder="When / where this kicks in." />
       </label>
+
+      <!-- Only on an existing rule: a rule being created has no past, and an
+           "Edit history — none" line on a blank form reads as a broken panel.
+           Keyed on ruleId so switching rules reloads rather than showing the
+           previous rule's history under the new one's text. -->
+      <RuleHistoryPanel
+        v-if="!isCreating && ruleId !== null"
+        :key="ruleId"
+        :rule-id="ruleId"
+        :current="store.currentRule"
+      />
     </aside>
   </div>
 </template>
diff --git a/frontend/src/components/rules/RuleHistoryPanel.vue b/frontend/src/components/rules/RuleHistoryPanel.vue
new file mode 100644
index 0000000..7abd1b9
--- /dev/null
+++ b/frontend/src/components/rules/RuleHistoryPanel.vue
@@ -0,0 +1,274 @@
+<script setup lang="ts">
+/**
+ * What a rule USED TO SAY — inside the slide-over, where a rule is read in
+ * full. Not on the list row: a history entry point there would compete with
+ * the row's actual job.
+ *
+ * A SIBLING OF HistoryPanel.vue, NOT A REUSE OF IT, and the reason is in its
+ * props: `noteId` + `currentBody`, a `NoteVersion` carrying tags and pin
+ * columns, a fetch of /api/notes/…, a `restore` emit, and pin/unpin buttons.
+ * Every one of those is note-shaped. Rules have no tags, no pins, and
+ * deliberately no restore, and a rule's text is EIGHT fields rather than one
+ * body — which changes the central question from "what changed" to "which
+ * fields moved".
+ *
+ * What was genuinely shared is shared: DiffView.vue takes DiffLine[] and
+ * nothing note-shaped, and the LCS walk now lives in utils/diff.ts, which
+ * this file uses rather than copying a fourth time (#3207).
+ */
+import { computed, onMounted, ref, watch } from "vue";
+import DiffView from "@/components/DiffView.vue";
+import { computeDiff } from "@/utils/diff";
+import {
+  listRuleVersions, getRuleVersion, type Rule, type RuleVersion,
+} from "@/api/rulebooks";
+import { useToastStore } from "@/stores/toast";
+
+const props = defineProps<{ ruleId: number; current: Rule | null }>();
+
+const toast = useToastStore();
+const versions = ref<RuleVersion[]>([]);
+const selected = ref<RuleVersion | null>(null);
+const expanded = ref(false);
+const loading = ref(false);
+const loadingDetail = ref(false);
+
+// The eight TEXT fields a version carries, in the order the editor shows
+// them. Narrowed to its own type rather than `keyof RuleVersion`, which would
+// also admit id/rule_id/user_id/created_at — none of which is text a reader
+// compares, and all of which would widen every lookup below to `number`.
+// Labels rather than column names: a reader is deciding whether to open a
+// row, and "How to apply" reads where "how_to_apply" has to be decoded.
+type TextField =
+  | "title" | "statement" | "when_to_apply" | "tier"
+  | "why" | "how_to_apply" | "verify_with" | "expires_when";
+
+const FIELDS: Array<[TextField, string]> = [
+  ["title", "Title"],
+  ["statement", "Statement"],
+  ["when_to_apply", "When to apply"],
+  ["tier", "Tier"],
+  ["why", "Why"],
+  ["how_to_apply", "How to apply"],
+  ["verify_with", "Check"],
+  ["expires_when", "Ends when"],
+];
+
+/**
+ * Which fields this edit moved.
+ *
+ * A version holds the text the edit REPLACED, so the edit is the step from
+ * this row to the NEXT NEWER state — the version above it in the list, or,
+ * for the newest row, the rule as it stands now. Comparing against the row
+ * below instead would attribute every change to the wrong edit.
+ */
+function changedFields(index: number): string[] {
+  const before = versions.value[index];
+  // `Rule` carries all eight as required strings; a RuleVersion carries them
+  // only once opened, which is what the undefined check below is about.
+  const after: Pick<Rule, TextField> | RuleVersion | null =
+    index === 0 ? props.current : versions.value[index - 1] ?? null;
+  if (!before || !after) return [];
+  return FIELDS
+    .filter(([key]) => {
+      // A listing row carries only the title; the rest arrive when opened.
+      // Undefined means NOT LOADED, which is not the same as unchanged — so a
+      // field nobody has fetched is claimed as neither.
+      const a = before[key];
+      const b = after[key];
+      if (a === undefined || b === undefined) return false;
+      return (a ?? "") !== (b ?? "");
+    })
+    .map(([, label]) => label);
+}
+
+/** True when this edit rewrote or removed the rule's check.
+ *
+ * Worth its own marker because editing `verify_with` silently drops
+ * `verified_at` (milestone 312) — the moment a rule re-entered the staleness
+ * sweep. That happens nowhere a reader can see it, and this row is the only
+ * surface that can say when it happened. */
+function checkChanged(index: number): boolean {
+  return changedFields(index).includes("Check");
+}
+
+const diff = computed(() => {
+  if (!selected.value || selected.value.statement === undefined) return [];
+  const now = props.current?.statement ?? "";
+  return computeDiff(now, selected.value.statement);
+});
+
+function stamp(iso: string): string {
+  return iso.slice(0, 10);
+}
+
+async function load() {
+  loading.value = true;
+  try {
+    versions.value = await listRuleVersions(props.ruleId);
+  } catch {
+    toast.show("Could not load this rule's history", "error");
+  } finally {
+    loading.value = false;
+  }
+}
+
+async function open(v: RuleVersion) {
+  if (selected.value?.id === v.id) {
+    selected.value = null;
+    return;
+  }
+  loadingDetail.value = true;
+  try {
+    const full = await getRuleVersion(props.ruleId, v.id);
+    // Merged back into the list so `changedFields` can compare against real
+    // text once a neighbour has been opened, instead of staying blind.
+    const at = versions.value.findIndex((x) => x.id === v.id);
+    if (at >= 0) versions.value[at] = { ...versions.value[at], ...full };
+    selected.value = versions.value[at] ?? full;
+  } catch {
+    toast.show("Could not open that version", "error");
+  } finally {
+    loadingDetail.value = false;
+  }
+}
+
+onMounted(load);
+watch(() => props.ruleId, () => { selected.value = null; load(); });
+</script>
+
+<template>
+  <section class="history">
+    <button class="toggle" :aria-expanded="expanded" @click="expanded = !expanded">
+      <span>Edit history</span>
+      <span class="count">{{ versions.length || "none" }}</span>
+    </button>
+
+    <div v-if="expanded" class="body">
+      <p v-if="loading" class="state">Loading…</p>
+
+      <!-- Never reworded is the ordinary case, and must not read as a fault. -->
+      <p v-else-if="!versions.length" class="state empty">
+        This rule has never been reworded. Nothing was recorded before the history
+        existed, so an older rule starts empty too.
+      </p>
+
+      <template v-else>
+        <p class="lede">
+          Each entry is what the rule said <em>before</em> that edit. The wording it
+          was changed to is the rule as it stands above.
+        </p>
+        <ol class="rows">
+          <li v-for="(v, i) in versions" :key="v.id" class="row">
+            <button
+              class="row-head"
+              :class="{ open: selected?.id === v.id }"
+              @click="open(v)"
+            >
+              <span class="when">{{ stamp(v.created_at) }}</span>
+              <span class="fields">
+                {{ changedFields(i).join(", ") || "opened to compare" }}
+              </span>
+              <span v-if="checkChanged(i)" class="check-moved">check reset</span>
+            </button>
+
+            <div v-if="selected?.id === v.id" class="detail">
+              <p v-if="loadingDetail" class="state">Loading…</p>
+              <template v-else>
+                <p v-if="checkChanged(i)" class="warn">
+                  This edit changed the rule's check, which cleared its verification
+                  stamp — the rule went back to the top of the staleness sweep here.
+                </p>
+                <dl class="fields-list">
+                  <template v-for="[key, label] in FIELDS" :key="key">
+                    <template v-if="key !== 'statement' && v[key]">
+                      <dt>{{ label }}</dt>
+                      <dd>{{ v[key] }}</dd>
+                    </template>
+                  </template>
+                </dl>
+                <h4>Statement</h4>
+                <DiffView v-if="diff.length" :diff="diff" />
+                <p v-else class="state">The statement did not change in this edit.</p>
+              </template>
+            </div>
+          </li>
+        </ol>
+      </template>
+    </div>
+  </section>
+</template>
+
+<style scoped>
+.history { border-top: 1px solid var(--fs-border-color); padding-top: var(--fs-space-3); }
+
+.toggle {
+  display: flex; align-items: center; gap: var(--fs-space-2); width: 100%;
+  background: none; border: none; padding: 0; cursor: pointer;
+  font: inherit; font-size: var(--fs-size-body-sm); color: var(--fs-text-secondary);
+}
+.toggle:hover { color: var(--fs-text-primary); }
+.count {
+  margin-left: auto; font-size: var(--fs-size-tiny); color: var(--fs-text-tertiary);
+  font-variant-numeric: tabular-nums;
+}
+
+.body { margin-top: var(--fs-space-3); display: flex; flex-direction: column; gap: var(--fs-space-3); }
+.state { margin: 0; font-size: var(--fs-size-body-sm); color: var(--fs-text-secondary); }
+.state.empty { color: var(--fs-text-tertiary); }
+.lede {
+  margin: 0; max-width: 62ch; font-size: var(--fs-size-tiny);
+  color: var(--fs-text-tertiary); line-height: var(--fs-leading-body);
+}
+
+.rows { list-style: none; margin: 0; padding: 0; display: flex; flex-direction: column; gap: var(--fs-space-2); }
+.row { background: var(--fs-surface-raised); border-radius: var(--fs-radius-md); }
+
+.row-head {
+  display: flex; align-items: baseline; gap: var(--fs-space-3); width: 100%;
+  background: none; border: none; cursor: pointer; text-align: left;
+  padding: var(--fs-space-2) var(--fs-space-3);
+  font: inherit; font-size: var(--fs-size-body-sm); color: var(--fs-text-primary);
+}
+.row-head:hover { background: var(--fs-surface-hover); border-radius: var(--fs-radius-md); }
+.when {
+  font-variant-numeric: tabular-nums; color: var(--fs-text-secondary);
+  font-size: var(--fs-size-tiny);
+}
+.fields { color: var(--fs-text-primary); min-width: 0; overflow-wrap: anywhere; }
+
+/* A TINT, not the solid token. `--fs-warning-fg` is defined as "warning text
+   ON A WARNING TINT" — painting it over solid `--fs-warning` is the same-hue
+   contrast failure #3141 records. The 12% mix is how theme.css builds its own
+   `-bg` pairs, and it keeps the value a resolvable var() rather than a raw hex
+   that check_design_tokens.py cannot see at all. */
+.check-moved {
+  margin-left: auto; flex: none;
+  background: color-mix(in srgb, var(--fs-warning) 12%, transparent);
+  color: var(--fs-warning-fg);
+  border-radius: var(--fs-radius-pill);
+  padding: 0.1rem 0.5rem;
+  font-size: var(--fs-size-tiny); letter-spacing: var(--fs-tracking-tiny);
+}
+
+.detail {
+  padding: 0 var(--fs-space-3) var(--fs-space-3);
+  display: flex; flex-direction: column; gap: var(--fs-space-2);
+}
+.warn {
+  margin: 0; font-size: var(--fs-size-tiny); line-height: var(--fs-leading-body);
+  color: var(--fs-warning-fg);
+  background: color-mix(in srgb, var(--fs-warning) 12%, transparent);
+  border-radius: var(--fs-radius-sm); padding: var(--fs-space-2);
+}
+.fields-list { display: grid; grid-template-columns: auto 1fr; gap: 0.15rem var(--fs-space-3); margin: 0; }
+.fields-list dt {
+  font-size: var(--fs-size-tiny); text-transform: uppercase;
+  letter-spacing: var(--fs-tracking-tiny); color: var(--fs-text-tertiary);
+}
+.fields-list dd {
+  margin: 0; font-size: var(--fs-size-body-sm);
+  color: var(--fs-text-primary); min-width: 0; overflow-wrap: anywhere;
+}
+h4 { margin: var(--fs-space-2) 0 0; font-size: var(--fs-size-tiny); color: var(--fs-text-tertiary); }
+</style>
diff --git a/frontend/src/composables/useAssist.ts b/frontend/src/composables/useAssist.ts
index 6ec14c7..5cad62d 100644
--- a/frontend/src/composables/useAssist.ts
+++ b/frontend/src/composables/useAssist.ts
@@ -1,4 +1,5 @@
 import { ref, computed, watch, type Ref } from "vue";
+import { computeDiff, type DiffLine } from "@/utils/diff";
 import { apiPost, apiPut, apiDelete, apiSSEStream, type SSEStreamHandle } from "@/api/client";
 import { useToastStore } from "@/stores/toast";
 import {
@@ -9,17 +10,16 @@ import {
 export type AssistState = "idle" | "streaming" | "review";
 export type ScopeMode = "document" | "section";
 
+// Re-exported: this composable was where DiffLine lived before the diff
+// moved to a shared util, and every consumer still imports the type from here.
+export type { DiffLine };
+
 export interface AssistTarget {
   text: string;
   startOffset: number;
   endOffset: number;
 }
 
-export interface DiffLine {
-  type: 'equal' | 'delete' | 'insert';
-  text: string;
-}
-
 export interface NoteDraft {
   id: number;
   note_id: number;
@@ -31,27 +31,6 @@ export interface NoteDraft {
   updated_at: string;
 }
 
-function computeDiff(a: string, b: string): DiffLine[] {
-  const aLines = a.split('\n');
-  const bLines = b.split('\n');
-  const m = aLines.length, n = bLines.length;
-  const dp: number[][] = Array.from({ length: m + 1 }, () => new Array(n + 1).fill(0));
-  for (let i = m - 1; i >= 0; i--)
-    for (let j = n - 1; j >= 0; j--)
-      dp[i][j] = aLines[i] === bLines[j]
-        ? dp[i+1][j+1] + 1
-        : Math.max(dp[i+1][j], dp[i][j+1]);
-  const result: DiffLine[] = [];
-  let i = 0, j = 0;
-  while (i < m && j < n) {
-    if (aLines[i] === bLines[j]) { result.push({ type: 'equal', text: aLines[i++] }); j++; }
-    else if (dp[i+1][j] >= dp[i][j+1]) result.push({ type: 'delete', text: aLines[i++] });
-    else result.push({ type: 'insert', text: bLines[j++] });
-  }
-  while (i < m) result.push({ type: 'delete', text: aLines[i++] });
-  while (j < n) result.push({ type: 'insert', text: bLines[j++] });
-  return result;
-}
 
 export function useAssist(body: Ref<string>, noteId?: Ref<number | null>, projectId?: Ref<number | null>) {
   const toast = useToastStore();
diff --git a/frontend/src/utils/diff.ts b/frontend/src/utils/diff.ts
new file mode 100644
index 0000000..eb47270
--- /dev/null
+++ b/frontend/src/utils/diff.ts
@@ -0,0 +1,50 @@
+/**
+ * Line diff — one copy, for every surface that shows what changed.
+ *
+ * WHY THIS FILE EXISTS. The same LCS walk was written out three times:
+ * privately in `useAssist.ts`, and again inside `HistoryPanel.vue` and
+ * `VersionHistorySection.vue`. The three were character-identical apart from
+ * quote style — nobody had diverged them on purpose, they were simply copied
+ * because `computeDiff` was never exported. Milestone 323 needed a fourth
+ * consumer (a rule's edit history), and a fourth copy is the cost #3207
+ * records: a fix or an improvement now has to be found in N places by someone
+ * who does not know N.
+ */
+
+export interface DiffLine {
+  type: "equal" | "delete" | "insert";
+  text: string;
+}
+
+/**
+ * Diff `a` against `b`, line by line.
+ *
+ * `delete` lines come from `a`, `insert` lines from `b` — so the caller
+ * decides which side reads as "before" by which argument it passes. Every
+ * caller here passes the CURRENT text as `a` and the older text as `b`, so a
+ * deletion is what the old version had and an insertion is what replaced it.
+ *
+ * O(m·n) in time and memory: fine for a note or a rule, and deliberately not
+ * generalised further, since nothing here diffs a file of thousands of lines.
+ */
+export function computeDiff(a: string, b: string): DiffLine[] {
+  const aLines = a.split("\n");
+  const bLines = b.split("\n");
+  const m = aLines.length, n = bLines.length;
+  const dp: number[][] = Array.from({ length: m + 1 }, () => new Array(n + 1).fill(0));
+  for (let i = m - 1; i >= 0; i--)
+    for (let j = n - 1; j >= 0; j--)
+      dp[i][j] = aLines[i] === bLines[j]
+        ? dp[i + 1][j + 1] + 1
+        : Math.max(dp[i + 1][j], dp[i][j + 1]);
+  const result: DiffLine[] = [];
+  let i = 0, j = 0;
+  while (i < m && j < n) {
+    if (aLines[i] === bLines[j]) { result.push({ type: "equal", text: aLines[i++] }); j++; }
+    else if (dp[i + 1][j] >= dp[i][j + 1]) result.push({ type: "delete", text: aLines[i++] });
+    else result.push({ type: "insert", text: bLines[j++] });
+  }
+  while (i < m) result.push({ type: "delete", text: aLines[i++] });
+  while (j < n) result.push({ type: "insert", text: bLines[j++] });
+  return result;
+}