feat(admin): follow presets on mltext fields per locale

- formState: changedLocales and presetUpdates hold the preset-follow rules
  for text and mltext pairs
- FormView applies presetUpdates on create, tracks per-locale hand edits of
  ML fields and follows the active ML locale
- rebuilt modules/boardwalk/dist

admin/src/components/form/formState.ts

@@ -92,6 +92,91 @@ export function presetValue(type: string, source: unknown): string {
     .replace(/^-+|-+$/g, '')
 }
 
+/** The locales whose text differs between two locale maps: every locale on
+ * either side, a missing key counting as ''. A value that is not a map counts
+ * as an empty map. */
+export function changedLocales(previous: unknown, next: unknown): string[] {
+  const before = localeRecord(previous)
+  const after = localeRecord(next)
+  const out: string[] = []
+  for (const locale of new Set([...Object.keys(before), ...Object.keys(after)])) {
+    if ((before[locale] ?? '') !== (after[locale] ?? '')) {
+      out.push(locale)
+    }
+  }
+  return out
+}
+
+/** Reports whether the administrator edited a preset target by hand: the
+ * whole field when no locale is given, one locale of an ML target otherwise. */
+export type PresetEdited = (target: string, locale?: string) => boolean
+
+function isMLType(type: string | undefined): boolean {
+  return type === 'mltext' || type === 'mlmarkdown'
+}
+
+/**
+ * The preset targets of `source` (fields.yaml `preset`) to rewrite after the
+ * source changed from `previous` to `next`: target name -> new value. Targets
+ * are text and mltext fields; a preset on any other type is ignored.
+ *
+ * - text -> text: the target takes presetValue of the source, until edited.
+ * - ML -> text: the target takes presetValue of the source's active-locale text.
+ * - text -> ML: only the target's active locale is written; its other locales stay.
+ * - ML -> ML: per locale. Each locale whose source text changed rewrites the
+ *   same locale of the target, unless the administrator edited that locale by
+ *   hand; the target's other locales stay.
+ *
+ * A preset never writes a locale the administrator neither typed in (source)
+ * nor sees (target), so untouched locales keep the server's fallback.
+ */
+export function presetUpdates(
+  fields: FormField[],
+  source: string,
+  previous: unknown,
+  next: unknown,
+  values: AdminRecord,
+  activeLocale: string,
+  isEdited: PresetEdited,
+): AdminRecord {
+  const out: AdminRecord = {}
+  const sourceML = isMLType(fields.find((field) => field.name === source)?.type)
+  for (const field of fields) {
+    const preset = field.preset
+    if (!preset || preset.field !== source || (field.type !== 'text' && field.type !== 'mltext')) {
+      continue
+    }
+    if (field.type === 'text') {
+      if (isEdited(field.name)) {
+        continue
+      }
+      out[field.name] = presetValue(preset.type, sourceML ? (localeRecord(next)[activeLocale] ?? '') : next)
+      continue
+    }
+    if (!sourceML) {
+      if (isEdited(field.name, activeLocale)) {
+        continue
+      }
+      out[field.name] = { ...localeRecord(values[field.name]), [activeLocale]: presetValue(preset.type, next) }
+      continue
+    }
+    const texts = localeRecord(next)
+    const target = { ...localeRecord(values[field.name]) }
+    let written = false
+    for (const locale of changedLocales(previous, next)) {
+      if (isEdited(field.name, locale)) {
+        continue
+      }
+      target[locale] = presetValue(preset.type, texts[locale] ?? '')
+      written = true
+    }
+    if (written) {
+      out[field.name] = target
+    }
+  }
+  return out
+}
+
 /** Initial values of a new record: schema defaults, toggles off, no ids.
  * ML fields seed an empty string for every enabled locale. */
 export function initialValues(fields: FormField[], enabledLocales: readonly string[] = []): AdminRecord {

admin/src/views/FormView.vue

@@ -1,5 +1,5 @@
 <script setup lang="ts">
-import { computed, nextTick, onBeforeUnmount, onMounted, provide, readonly, ref, watchEffect } from 'vue'
+import { computed, nextTick, onBeforeUnmount, onMounted, provide, readonly, ref, watch, watchEffect } from 'vue'
 import { onBeforeRouteLeave, onBeforeRouteUpdate, useRoute, useRouter } from 'vue-router'
 import { ArrowLeft, Trash2 } from '@lucide/vue'
 import { api } from '../api/client'
@@ -17,6 +17,8 @@ import { FORM_ASSETS, FORM_ENABLED_LOCALES, FORM_LOCALE, FORM_PATCH, FORM_SESSIO
 import { RELATION_MANAGER, needsRecord } from '../components/form/registry'
 import {
   DEFAULT_TAB,
+  ML_LOCALE_CHANGE,
+  changedLocales,
   contextAllows,
   editablePayload,
   fieldErrors,
@@ -24,7 +26,7 @@ import {
   initialValues,
   mergeMLValue,
   panelDomId,
-  presetValue,
+  presetUpdates,
   schemaEnabledLocales,
   snapshot,
   tabDomId,
@@ -205,24 +207,60 @@ async function load(): Promise<void> {
     values.value = initialValues(fields.value, schemaEnabledLocales(schema.value.meta))
   }
   adopt(recordResult?.data)
+  mlLocale.value = firstMLLocale()
   activeTab.value = tabs.value[0]?.key ?? DEFAULT_TAB
   loading.value = false
 }
 
-// Preset fields (fields.yaml `preset`, UI-SPEC S7): on create, a text field
-// follows its source field until the administrator edits it by hand; that
-// stops it for as long as the form is open. On update nothing follows.
+// Preset fields (fields.yaml `preset`, UI-SPEC S7): on create, a text or
+// mltext field follows its text or mltext source until the administrator
+// edits it by hand; that stops it for as long as the form is open. An mltext
+// pair follows per locale and a hand edit of one locale stops only that
+// locale; a mixed pair follows the active ML locale (presetUpdates). On
+// update nothing follows.
 const edited = new Set<string>()
+// The locales of each ML field the administrator edited by hand.
+const editedLocales = new Map<string, Set<string>>()
+// The locale every ML control shows (MLTextField broadcasts the choice).
+const mlLocale = ref('en')
+
+function firstMLLocale(): string {
+  return schemaEnabledLocales(schema.value?.meta)[0] ?? 'en'
+}
+
+function onMLLocale(event: Event): void {
+  const locale = (event as CustomEvent<unknown>).detail
+  if (typeof locale === 'string' && locale !== '') {
+    mlLocale.value = locale
+  }
+}
+
+// FormGrid remounts the new panel's ML controls on the first enabled locale.
+watch(activeTab, () => {
+  mlLocale.value = firstMLLocale()
+})
+
+function isEdited(target: string, locale?: string): boolean {
+  return locale === undefined ? edited.has(target) : (editedLocales.get(target)?.has(locale) ?? false)
+}
 
 function update(name: string, value: unknown): void {
+  const previous = values.value[name]
   const next = { ...values.value, [name]: value }
   edited.add(name)
-  if (mode === 'create') {
-    for (const field of fields.value) {
-      if (field.preset?.field === name && field.type === 'text' && !edited.has(field.name)) {
-        next[field.name] = presetValue(field.preset.type, value)
-      }
+  const type = fields.value.find((field) => field.name === name)?.type
+  if (type === 'mltext' || type === 'mlmarkdown') {
+    let locales = editedLocales.get(name)
+    if (!locales) {
+      locales = new Set<string>()
+      editedLocales.set(name, locales)
     }
+    for (const locale of changedLocales(previous, value)) {
+      locales.add(locale)
+    }
+  }
+  if (mode === 'create') {
+    Object.assign(next, presetUpdates(fields.value, name, previous, value, values.value, mlLocale.value, isEdited))
   }
   values.value = next
   if (errors.value[name]) {
@@ -411,8 +449,14 @@ function onBeforeUnload(event: BeforeUnloadEvent): void {
   }
 }
 
-onMounted(() => window.addEventListener('beforeunload', onBeforeUnload))
-onBeforeUnmount(() => window.removeEventListener('beforeunload', onBeforeUnload))
+onMounted(() => {
+  window.addEventListener('beforeunload', onBeforeUnload)
+  window.addEventListener(ML_LOCALE_CHANGE, onMLLocale)
+})
+onBeforeUnmount(() => {
+  window.removeEventListener('beforeunload', onBeforeUnload)
+  window.removeEventListener(ML_LOCALE_CHANGE, onMLLocale)
+})
 
 // The header's last breadcrumb is this record's title (design: Top header).
 watchEffect(() => setRecordCrumb(loading.value ? '' : title.value))