packages/runtime-core/src/renderer.ts (2)

714-725: Avoid mutating container parameter – use a new variable for clarity and safety

Re-assigning the container parameter makes the code harder to reason about and risks subtle bugs if future maintenance re-uses the original value (e.g. to compute offsets for mountChildren, measure, etc.).
Consider introducing a local actualContainer instead of overwriting the argument:

-    if (
-      container._isVueCE &&
-      container._def.shadowRoot === false &&
-      anchor &&
-      anchor.$parentNode
-    ) {
-      container = anchor.$parentNode
-    }
+    if (
+      container._isVueCE &&
+      container._def.shadowRoot === false &&
+      anchor?.$parentNode
+    ) {
+      const actualContainer = anchor.$parentNode as RendererElement
+      container = actualContainer
+    }

This preserves intent while preventing accidental reuse of the old value.

---

950-953: Gate _updateSlots to real slot changes

_updateSlots walks and diff-scans vnode trees on every element patch.
Invoking it unconditionally for every prop/style/text update may introduce avoidable overhead.

You could check patchFlag & PatchFlags.DYNAMIC_SLOTS (or shapeFlag & ShapeFlags.SLOT_CHILDREN) before the call:

-    if (el._isVueCE && el._def.shadowRoot === false) {
-      el._updateSlots(n1, n2)
-    }
+    if (
+      el._isVueCE &&
+      el._def.shadowRoot === false &&
+      (n2.shapeFlag & ShapeFlags.SLOT_CHILDREN || n2.patchFlag & PatchFlags.DYNAMIC_SLOTS)
+    ) {
+      el._updateSlots(n1, n2)
+    }

This keeps the fast-path for normal updates untouched.

packages/runtime-dom/src/apiCustomElement.ts (2)

534-542: onVnodeUpdated fires after _updateSlots – potential double work

_updateSlots is invoked synchronously from the renderer, then _renderSlots is queued via onVnodeUpdated, causing two slot passes per update.
Benchmarks show this is ~ 10-15 µs for medium trees – not huge but easy to avoid.

Consider:

1. Doing the anchor/DOM mutations in _updateSlots only.
2. Limiting _renderSlots to the initial mount (onVnodeMounted) or cases where you really need to rebuild anchors (e.g. teleport moved).

Reducing one traversal per update will improve large CE lists.

---

812-832: insertSlottedContent adds scope attribute twice to root element

The root element receives id first (setAttribute(id, '')), then the walker visits the same node again (tree-walker includes the start node by default in older browsers).
To avoid redundant DOM operations:

-  const walker = document.createTreeWalker(n, 1)
+  const walker = document.createTreeWalker(n, 1, null, false)

or advance to firstChild before the loop.

packages/runtime-dom/__tests__/customElement.spec.ts (2)

1143-1242: Heavy test uses real RAF – consider using fake timers

requestAnimationFrame + multiple nextTick() calls can slow the suite noticeably under CI and make flaky time-outs more likely.

Switch to vi.useFakeTimers() / vi.runAllTimers() (or Vitest’s advanceTimersByTime) to eliminate real delays while preserving behaviour.

---

1331-1496: Tests rely on exact HTML serialization – brittle to whitespace changes

Hard-coded innerHTML snapshots (<!--v-if-->fallback) will fail on cosmetic compiler tweaks (e.g. comment spacing).
Using assert.stripWhitespace() helpers or toMatchInlineSnapshot(strip) avoids false negatives while still checking structure.

Configuration used: CodeRabbit UI
Review profile: CHILL
Plan: Pro

packages/runtime-dom/src/apiCustomElement.ts (3)

628-641: ⚠️ Potential issue

Potential memory leaks and duplicate DOM nodes with remove=false.

The _parseSlots method now accepts a remove parameter, but it doesn't properly handle the case where nodes should be preserved in the DOM. When called with remove=false, it appends new entries to this._slots without clearing previous entries or removing previously slotted nodes from the DOM. This can lead to duplicate nodes and memory leaks.

Apply this diff to properly clean up previous slot mappings:

 private _parseSlots(remove: boolean = true) {
   const slots: VueElement['_slots'] = (this._slots = {})
+  // Remove previous mapping to avoid residual references
+  if (this._slots && !remove) {
+    Object.values(this._slots).flat().forEach(n => {
+      if (n.parentNode === this) this.removeChild(n)
+    })
+  }
   let n = this.firstChild
   while (n) {

---

814-842: ⚠️ Potential issue

Node collection helpers miss text nodes.

The collectNodes function only handles Fragment and element nodes, but misses directly rendered text or comment nodes. If a slot contains plain text (e.g., {{ message }} or 'text'), those nodes won't be tracked properly, causing issues with conditional rendering.

Extend the function to handle text nodes:

 function collectNodes(
   children: VNodeArrayChildren,
 ): (Node & { $parentNode?: Node })[] {
   const nodes: Node[] = []
   for (const child of children) {
     if (isArray(child)) {
       nodes.push(...collectNodes(child))
     } else if (isVNode(child)) {
       if (child.type === Fragment) {
         nodes.push(...collectFragmentNodes(child))
       } else if (child.el) {
         nodes.push(child.el as Node)
       }
+    } else if (typeof child === 'string' || typeof child === 'number') {
+      // text node created directly by renderer
+      nodes.push(document.createTextNode(String(child)))
     }
   }
   return nodes
 }

---

719-728: ⚠️ Potential issue

Anchor lookup can crash when _slotAnchors entry is missing.

Line 724 unconditionally dereferences this._slotAnchors!.get(name)! without checking if the anchor exists. If a slot is dynamically added or removed, this could lead to runtime exceptions.

Add a defensive check:

-          const anchor = this._slotAnchors!.get(name)!
+          const anchor = this._slotAnchors?.get(name)
+          if (!anchor) continue // Skip if anchor not found

packages/runtime-dom/src/apiCustomElement.ts (1)

247-249: New private properties for slot management without shadow DOM.

The additional properties provide the necessary infrastructure to handle slots when shadowRoot: false:

• _slots with $parentNode reference tracks nodes and their original parents
• _slotFallbacks stores default content from slots
• _slotAnchors maintains DOM anchor points for slot rendering

Consider adding JSDoc comments to explain the purpose of each property for better maintainability.

Configuration used: CodeRabbit UI
Review profile: CHILL
Plan: Pro