fix(custom-element): enhance slot handling with `shadowRoot:false`

[edison1105]

close #13206 close #13234

Note This PR only fixed bugs in the optimized mode, while issues still exist in the full diff scenario. This happens because _renderSlots only renders slots that have corresponding outlets, meaning not all slots in this._slots are rendered to the DOM tree. During full diff, parent.insertBefore(node, anchor) throws an error because the anchor node isn't actually in the DOM tree.

I couldn't find a suitable solution, so rolled back the full diff related code in 3e724095072d6ae4f8b095e6fc0eb25eb1564f67

Summary by CodeRabbit

Summary by CodeRabbit

• New Features
  • Enhanced support for Vue custom elements without Shadow DOM, improving slot fallback content management and dynamic slot updates.
• Bug Fixes
  • Fixed element mounting and slot updating issues for custom elements with shadowRoot: false, ensuring correct DOM insertion and slot rendering.
• Tests
  • Added extensive tests covering slot updates, fallback content, and conditional rendering in custom elements without Shadow DOM for improved stability.

[github-actions[bot]]

Size Report

Bundles

File	Size	Gzip	Brotli
runtime-dom.global.prod.js	102 kB (+1.45 kB)	38.6 kB (+412 B)	34.8 kB (+358 B)
vue.global.prod.js	160 kB (+1.45 kB)	58.8 kB (+404 B)	52.3 kB (+378 B)

Usages

Name	Size	Gzip	Brotli
createApp (CAPI only)	46.6 kB (+74 B)	18.2 kB (+38 B)	16.7 kB (+31 B)
createApp	54.6 kB (+74 B)	21.2 kB (+39 B)	19.4 kB (+41 B)
createSSRApp	58.8 kB (+74 B)	23 kB (+34 B)	21 kB (+40 B)
defineCustomElement	60.7 kB (+1.45 kB)	23.2 kB (+426 B)	21.1 kB (+338 B)
overall	68.6 kB (+74 B)	26.4 kB (+41 B)	24 kB (+41 B)

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commit: 3e72409

[wolandec]

Hi @edison1105, thanks for this fix, it works well with Vue 3.

However it doesn't work if we create custom elements from Vue 3 components and use them in Vue 2 or React wrapping with Vue 2 or React component similarly as I did in reproduction playground (see CEWrapperOne) . To provide more context: here is a schema of what we are working on at the moment. So we are creating a component library based on Vue 3 but to reuse this implementation in different applications written in Vue 2 or React we use custom elements created from Vue 3 components implementation.

[edison1105]

@wolandec I also found some issues in other scenarios and haven't found a good solution yet, so the PR is still in Draft status.

[wolandec]

@edison1105 thanks for the clarification, I'll prepare a reproduction repo for our case if it helps.

[wolandec]

@edison1105 https://github.com/wolandec/vue-core-issue-13206 Here is the repository with the reproduction of our case of using, I hope it helps. Please let me know if you need any help, thank you!

[coderabbitai[bot]]

"""

Walkthrough

The changes introduce enhanced support for Vue custom elements using shadowRoot: false, focusing on correct slot and conditional rendering behavior. Core renderer logic, slot parsing, and DOM patching are updated, and new tests are added to verify slot fallback and v-if/v-show handling in custom elements without shadow DOM.

Changes

File(s)	Change Summary
packages/runtime-core/src/renderer.ts	Modified element mounting and patching logic to handle Vue custom elements with shadowRoot: false, including container reassignment, slot updates via _updateSlots, and fallback handling in patchBlockChildren.
packages/runtime-dom/src/apiCustomElement.ts	Enhanced slot management for custom elements without shadow DOM: introduced _slotFallbacks, _slotAnchors, and modified _slots properties; added _updateSlots, _parseSlotFallbacks methods; improved fallback slot capturing and rendering; adjusted slot parsing to retain parent references; added helpers for DOM insertion and node collection; updated cleanup in disconnectedCallback.
packages/runtime-dom/tests/customElement.spec.ts	Added tests for custom elements with shadowRoot: false to verify correct slot fallback, conditional rendering (v-if), and slot content switching, both with optimized and standard rendering modes. No changes to existing logic.

Sequence Diagram(s)

sequenceDiagram
    participant Host as Host App
    participant VueCE as Vue Custom Element (shadowRoot: false)
    participant Renderer as Renderer
    participant DOM as DOM

    Host->>VueCE: Insert slotted content (with v-if/v-show)
    VueCE->>Renderer: Mount element
    Renderer->>VueCE: Call _parseSlotFallbacks and _renderSlots
    VueCE->>DOM: Insert anchor and slot/fallback content
    Host->>VueCE: Update reactive state (e.g., v-if toggles)
    VueCE->>Renderer: Patch element
    Renderer->>VueCE: Call _updateSlots (oldVNode, newVNode)
    VueCE->>DOM: Update slot content or fallback as needed

Assessment against linked issues

Objective	Addressed	Explanation
Correctly render and update slotted content with v-if/v-show in custom elements ( #13206 )	✅
Prevent errors and ensure named slot content with v-if works without exceptions ( #13234 )	✅

Poem

In the garden of slots, where the shadows don’t grow,
The v-if and v-show now shimmer and glow.
Fallbacks and anchors, a bunny’s delight,
No more null errors in the custom element night!
🐇✨
Hop-hop—slots fixed just right! """

[!TIP]

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[edison1105]

/ecosystem-ci run

[vue-bot]

📝 Ran ecosystem CI: Open

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