fix(transition): get root el of DEV_FRAGMENT_ROOT

[jonaskuske]

fix #6745

___

an alternative to re-assigning el would be something like

prevChildren = prevChildren.map((child) => {
  if (DEV_ROOT_FRAGMENT) {
    const clone = cloneVNode(child)
    clone.el = elementRoot.el
    return clone
  }
  return child
})

but I don't think the cloning is actually necessary here. Not sure though.

[github-actions]

Size Report

Bundles

File	Size	Gzip	Brotli
runtime-dom.global.prod.js	90.5 kB (+27 B)	34.4 kB (+33 B)	31 kB (+42 B)
vue.global.prod.js	148 kB (+27 B)	53.7 kB (+22 B)	47.9 kB (+12 B)

Usages

Name	Size	Gzip	Brotli
createApp	50.7 kB	19.8 kB	18.1 kB
createSSRApp	54.1 kB	21.1 kB	19.3 kB
defineCustomElement	53 kB	20.6 kB	18.7 kB
overall	64.4 kB	24.8 kB	22.5 kB

[skirtles-code]

I think this may have been fixed in 3.4.22 by #9421.