Fix ParentNode insertBefore links

Author: TrevorSundberg

.changeset/young-owls-kick.md

@@ -0,0 +1,5 @@
+---
+'@remote-dom/polyfill': patch
+---
+
+Fix linked-list corruption when inserting a child before a non-head sibling.

packages/polyfill/source/ParentNode.ts

@@ -124,6 +124,7 @@ export class ParentNode extends ChildNode {
       child[NEXT] = before;
       child[PREV] = before[PREV];
       if (before[PREV] === null) this[CHILD] = child;
+      else before[PREV][NEXT] = child;
       before[PREV] = child;
     } else {
       child[NEXT] = null;

packages/polyfill/source/tests/parent-node.test.ts

@@ -0,0 +1,63 @@
+import {Window} from '../index.ts';
+
+import {describe, it, expect, beforeEach} from 'vitest';
+
+describe('ParentNode', () => {
+  beforeEach(() => {
+    const window = new Window();
+    Window.setGlobalThis(window);
+  });
+
+  it('updates sibling links when inserting before a non-head child', () => {
+    const parent = document.createElement('div');
+    const first = document.createElement('first');
+    const second = document.createElement('second');
+    const third = document.createElement('third');
+    const inserted = document.createElement('inserted');
+
+    parent.append(first, second, third);
+    parent.insertBefore(inserted, second);
+
+    expect([...parent.childNodes]).toStrictEqual([
+      first,
+      inserted,
+      second,
+      third,
+    ]);
+    expect(nodesFromSiblingLinks(parent)).toStrictEqual([...parent.childNodes]);
+    expect(first.nextSibling).toBe(inserted);
+    expect(inserted.previousSibling).toBe(first);
+    expect(inserted.nextSibling).toBe(second);
+    expect(second.previousSibling).toBe(inserted);
+  });
+
+  it('updates sibling links when moving a child before a non-head child', () => {
+    const parent = document.createElement('div');
+    const first = document.createElement('first');
+    const second = document.createElement('second');
+    const third = document.createElement('third');
+    const moved = document.createElement('moved');
+
+    parent.append(first, second, third, moved);
+    parent.insertBefore(moved, second);
+
+    expect([...parent.childNodes]).toStrictEqual([first, moved, second, third]);
+    expect(nodesFromSiblingLinks(parent)).toStrictEqual([...parent.childNodes]);
+    expect(first.nextSibling).toBe(moved);
+    expect(moved.previousSibling).toBe(first);
+    expect(moved.nextSibling).toBe(second);
+    expect(second.previousSibling).toBe(moved);
+  });
+});
+
+function nodesFromSiblingLinks(parent: Node) {
+  const nodes: Node[] = [];
+  let node = parent.firstChild;
+
+  while (node) {
+    nodes.push(node);
+    node = node.nextSibling;
+  }
+
+  return nodes;
+}