fix(core): ensure pseudo shadow DOM getters return null instead of un…

src/runtime/dom-extras.ts

@@ -396,14 +396,14 @@ export const patchChildSlotNodes = (elm: HTMLElement) => {
   patchHostOriginalAccessor('firstChild', elm);
   Object.defineProperty(elm, 'firstChild', {
     get() {
-      return this.childNodes[0];
+      return this.childNodes[0] || null;
     },
   });
 
   patchHostOriginalAccessor('lastChild', elm);
   Object.defineProperty(elm, 'lastChild', {
     get() {
-      return this.childNodes[this.childNodes.length - 1];
+      return this.childNodes[this.childNodes.length - 1] || null;
     },
   });
 
@@ -459,7 +459,7 @@ const patchNextSibling = (node: Node) => {
       const parentNodes = this['s-ol']?.parentNode.childNodes;
       const index = parentNodes?.indexOf(this);
       if (parentNodes && index > -1) {
-        return parentNodes[index + 1];
+        return parentNodes[index + 1] || null;
       }
       return this.__nextSibling;
     },
@@ -480,7 +480,7 @@ const patchNextElementSibling = (element: Element) => {
       const parentEles = this['s-ol']?.parentNode.children;
       const index = parentEles?.indexOf(this);
       if (parentEles && index > -1) {
-        return parentEles[index + 1];
+        return parentEles[index + 1] || null;
       }
       return this.__nextElementSibling;
     },
@@ -501,7 +501,7 @@ const patchPreviousSibling = (node: Node) => {
       const parentNodes = this['s-ol']?.parentNode.childNodes;
       const index = parentNodes?.indexOf(this);
       if (parentNodes && index > -1) {
-        return parentNodes[index - 1];
+        return parentNodes[index - 1] || null;
       }
       return this.__previousSibling;
     },
@@ -523,7 +523,7 @@ const patchPreviousElementSibling = (element: Element) => {
       const index = parentNodes?.indexOf(this);
 
       if (parentNodes && index > -1) {
-        return parentNodes[index - 1];
+        return parentNodes[index - 1] || null;
       }
       return this.__previousElementSibling;
     },
@@ -541,7 +541,7 @@ export const patchParentNode = (node: Node) => {
   patchHostOriginalAccessor('parentNode', node);
   Object.defineProperty(node, 'parentNode', {
     get: function () {
-      return this['s-ol']?.parentNode || this.__parentNode;
+      return this['s-ol']?.parentNode || this.__parentNode || null;
     },
     set: function (value) {
       // mock-doc sets parentNode?

src/runtime/test/dom-extras.spec.tsx

@@ -140,4 +140,34 @@ describe('dom-extras - patches for non-shadow dom methods and accessors', () =>
     expect(specPage.root.children[0].__parentNode.tagName).toBe('DIV');
     expect(specPage.root.childNodes[0].__parentNode.tagName).toBe('DIV');
   });
+
+  it('returns null (not undefined) for out-of-bounds siblings/children', async () => {
+    specPage.root.childNodes.forEach((node: Node) => patchSlottedNode(node));
+
+    const firstSlottedNode = specPage.root.childNodes[0];
+    expect(firstSlottedNode.previousSibling).toBeNull();
+
+    const lastSlottedNode = specPage.root.childNodes[specPage.root.childNodes.length - 1];
+    expect(lastSlottedNode.nextSibling).toBeNull();
+
+    const firstSlottedElement = specPage.root.children[0];
+    expect(firstSlottedElement.previousElementSibling).toBeNull();
+
+    const lastSlottedElement = specPage.root.children[specPage.root.children.length - 1];
+    expect(lastSlottedElement.nextElementSibling).toBeNull();
+  });
+
+  it('returns null (not undefined) for firstChild/lastChild on empty patched hosts', async () => {
+    const emptyPage = await newSpecPage({
+      components: [],
+      html: `<div id="empty"></div>`,
+    });
+    const emptyElm = emptyPage.root;
+    (emptyElm as any).__childNodes = [];
+    patchPseudoShadowDom(emptyElm);
+
+    expect(emptyElm.firstChild).toBeNull();
+    expect(emptyElm.lastChild).toBeNull();
+  });
 });
+