fix: only inits form association if element is form associated

src/mutation-observers.ts

@@ -39,8 +39,9 @@ export function observerCallback(mutationList: MutationRecord[]) {
         const formElements = formElementsMap.get(node as unknown as HTMLFormElement);
         const walker = document.createTreeWalker(node, NodeFilter.SHOW_ELEMENT, {
           acceptNode(node: ICustomElement): number {
-            return internalsMap.has(node) && !formElements && !formElements.has(node) ?
-              NodeFilter.FILTER_ACCEPT : NodeFilter.FILTER_SKIP;
+            return (
+              internalsMap.has(node) && node.constructor['formAssociated'] && !formElements && !formElements.has(node)
+            ) ? NodeFilter.FILTER_ACCEPT : NodeFilter.FILTER_SKIP;
           }
         });
 

test/ElementInternals.test.js

@@ -181,6 +181,7 @@ describe('The ElementInternals polyfill', () => {
       constructor() {
         super();
         this.internals = this.attachInternals();
+        this.internals.role = 'generic';
       }
     }
 
@@ -199,6 +200,16 @@ describe('The ElementInternals polyfill', () => {
     it('will throw from setFormValue', async () => {
       expect(() => element.internals.setFormValue('foo')).to.throw();
     });
+
+    describe('inside a form', () => {
+      let form;
+
+      afterEach(async () => await fixtureCleanup(form));
+
+      it('will not throw', async () => {
+        form = await fixture(html`<form><not-associated></not-associated></form>`);
+      })
+    })
   });
 
   describe('inside a custom element with a form', () => {