fix(field): add form elements in the correct order

packages/field/src/FormRegistrarMixin.js

@@ -63,13 +63,17 @@ export const FormRegistrarMixin = dedupeMixin(
         this.__hasBeenRendered = true;
       }
 
-      addFormElement(child) {
+      addFormElement(child, index) {
         // This is a way to let the child element (a lion-fieldset or lion-field) know, about its parent
         // eslint-disable-next-line no-param-reassign
         child.__parentFormGroup = this;
 
+        if (index > 0) {
+          this.formElements.splice(index, 0, child);
+        } else {
           this.formElements.push(child);
         }
+      }
 
       removeFormElement(child) {
         const index = this.formElements.indexOf(child);
@@ -89,7 +93,14 @@ export const FormRegistrarMixin = dedupeMixin(
           return;
         }
         ev.stopPropagation();
-        this.addFormElement(child);
+
+        // Check for siblings to determine the right order to insert into formElements
+        // If there is no next sibling, index is -1
+        let indexToInsertAt = -1;
+        if (this.formElements && Array.isArray(this.formElements)) {
+          indexToInsertAt = this.formElements.indexOf(child.nextElementSibling);
+        }
+        this.addFormElement(child, indexToInsertAt);
       }
 
       _onRequestToRemoveFormElement(ev) {

packages/field/test-suites/FormRegistrationMixins.suite.js

@@ -168,6 +168,44 @@ export const runRegistrationSuite = customConfig => {
         expect(el.formElements.length).to.equal(1);
       });
 
+      it('adds elements to formElements in the right order', async () => {
+        const el = await fixture(html`
+          <${parentTag}>
+            <${childTag}></${childTag}>
+            <${childTag}></${childTag}>
+            <${childTag}></${childTag}>
+          </${parentTag}>
+        `);
+
+        expect(el.formElements.length).to.equal(3);
+
+        // In the middle
+        const secondChild = el.firstElementChild.nextElementSibling;
+        const newField = await fixture(html`
+          <${childTag}></${childTag}>
+        `);
+        secondChild.insertAdjacentElement('beforebegin', newField);
+
+        expect(el.formElements.length).to.equal(4);
+        expect(el.formElements[1]).dom.to.equal(newField);
+
+        // Prepending
+        const anotherField = await fixture(html`
+          <${childTag}></${childTag}>
+        `);
+        el.prepend(anotherField);
+        expect(el.formElements.length).to.equal(5);
+        expect(el.formElements[0]).dom.to.equal(anotherField);
+
+        // Appending
+        const yetAnotherField = await fixture(html`
+          <${childTag}></${childTag}>
+        `);
+        el.appendChild(yetAnotherField);
+        expect(el.formElements.length).to.equal(6);
+        expect(el.formElements[5]).dom.to.equal(anotherField);
+      });
+
       // find a proper way to do this on polyfilled browsers
       it.skip('fires event "form-element-register" with the child as ev.target', async () => {
         const registerSpy = sinon.spy();