fix: render custom element arrays as a Fragment to avoid remounting on re-render

[drubot]

Closes #530.

Problem

renderCustomElements() is documented for <component :is="renderCustomElements(page.content)" />, which Vue re-evaluates on every render of the consuming component. For an array payload it returned a freshly created anonymous component on each call, so <component :is> saw a new component type every render and unmounted then remounted the entire custom-element subtree on any unrelated re-render — discarding component state, focus and scroll position. Single-element payloads (returned as a plain VNode) were unaffected, which makes the bug easy to miss.

Fix

Wrap the VNode[] in a Fragment instead of a new defineComponent:

if (Array.isArray(vnodes)) {
  return h(Fragment, vnodes)
}

A Fragment has a stable type, so a re-render patches the children in place rather than remounting, while still reflecting content changes. <component :is> accepts a VNode, so the documented template usage is unchanged and the rendered markup is identical (a Fragment emits its children with no wrapper element). The return type narrows from VNode | Component | null to VNode | null.

Test

Adds a regression test to test/nuxt/renderCustomElements.test.ts that renders an array payload the documented way, bumps an unrelated ref to force a parent re-render, and asserts a mount-counting child is not remounted. It fails on the old code (mount count 2) and passes with this fix (mount count 1). The existing array-rendering assertions (markup output, empty arrays) are unchanged.

Notes

Drafted with assistance from Claude Code.

[fago]

I verified the test fails with the fix, good improvement.