test(16.2-06): pin the untouched-value skip in cabana and PluginField

- TestFieldTypeUntouched: served values skip Decode, writes and notices;
  one changed locale writes and reports only that locale; canonical JSON
  comparison; a create never skips
- PluginField: an element that echoes its value on connect is never
  written a second time

admin/tests/form/PluginField.test.ts

@@ -245,6 +245,81 @@ describe('stored value reload', () => {
   })
 })
 
+/** A stub element that re-emits its current value when it connects, as an
+ * element that normalizes nothing but announces its value on mount does. */
+class AcmeDemoEcho extends AcmeDemoNotes {
+  connectedCallback(): void {
+    this.dispatchEvent(new CustomEvent(FIELD_CHANGE_EVENT, { bubbles: true, detail: { value: this.value } }))
+  }
+}
+if (!customElements.get('acme-demo-echo')) {
+  customElements.define('acme-demo-echo', AcmeDemoEcho)
+}
+
+describe('untouched value (D-03)', () => {
+  // The form: every update:modelValue becomes the next modelValue, as
+  // FormView's v-model does.
+  function mountEcho(modelValue: unknown) {
+    const wrapper: VueWrapper = mount(PluginField, {
+      props: {
+        field: notesField({ element: 'acme-demo-echo' }),
+        modelValue,
+        controlId: 'f-notes',
+        recordId: 7,
+        source: SOURCE,
+        'onUpdate:modelValue': (value: unknown) => wrapper.setProps({ modelValue: value }),
+      },
+      global: { provide: { [FORM_LOCALE as symbol]: ref('pl') } },
+      // Attached, so the element's connectedCallback runs.
+      attachTo: document.body,
+    })
+    return wrapper
+  }
+
+  function echoElement(wrapper: VueWrapper): AcmeDemoEcho {
+    const found = wrapper.find('acme-demo-echo').element
+    if (!(found instanceof AcmeDemoEcho)) {
+      throw new Error('echo element not mounted')
+    }
+    return found
+  }
+
+  it('keeps the served HTML identical when the element echoes it on connect', async () => {
+    const served = '\n<!-- imported -->\n<p>Grzyby <b>rosną</b></p>\n<footer class="author-footer"><p>Autor</p></footer>\n'
+    const wrapper = mountEcho(served)
+    await flushPromises()
+    const el = echoElement(wrapper)
+    expect(wrapper.emitted('update:modelValue')).toEqual([[served]])
+    expect((wrapper.props() as { modelValue: unknown }).modelValue).toBe(served)
+    expect(el.value).toBe(served)
+    // The property was written once, before connect; the echo wrote nothing.
+    expect(el.writes).toEqual([served])
+  })
+
+  it('does not write an equal object value a second time', async () => {
+    const served = { blocks: [{ type: 'p', text: 'a' }], version: 1 }
+    const wrapper = mountEcho(served)
+    await flushPromises()
+    const el = echoElement(wrapper)
+    expect((wrapper.props() as { modelValue: unknown }).modelValue).toEqual(served)
+    // An equal copy from the server (a reload) is not written either.
+    await wrapper.setProps({ modelValue: JSON.parse(JSON.stringify(served)) })
+    await flushPromises()
+    expect(el.writes).toEqual([served])
+  })
+
+  it('writes a value that really differs exactly once', async () => {
+    const wrapper = mountEcho('<p>a</p>')
+    await flushPromises()
+    const el = echoElement(wrapper)
+    await wrapper.setProps({ modelValue: '<p>b</p>' })
+    await flushPromises()
+    await wrapper.setProps({ modelValue: '<p>b</p>' })
+    await flushPromises()
+    expect(el.writes).toEqual(['<p>a</p>', '<p>b</p>'])
+  })
+})
+
 describe('MLPluginField', () => {
   class AcmeDemoNotesml extends AcmeDemoNotes {}
   if (!customElements.get('acme-demo-notesml')) {