Wrapper units can expand from paragraph plus list or quote to paragraph plus list plus quote without a new insert seam

What happened

After proving list-item units with:

• paragraph + nested-list
• paragraph + quote

the next honest widening step was:

• paragraph + nested-list + quote

What fixed it

It did not need a new insertion model.

The same sibling-unit seam held:

• if the target container is a bulleted-list
• and the fragment being inserted is a list-item
• keep treating it as a sibling unit

That was enough for:

• slate-v2 contracts
• slate-dom-v2 clipboard round-trip
• real Chromium copy/paste routes

Why this works

The important question is still not the exact child-block mix.

It is:

• does the fragment still behave as one wrapper unit at insert time?

For paragraph + nested-list + quote, yes.

The browser/runtime stack still decides the final landing point, but the underlying unit-insert seam does not need to change.

Reusable rule

When widening wrapper-unit geometry:

• expand the child-block mix one step at a time
• keep the existing sibling-unit seam if the inserted fragment is still one real wrapper unit
• only look for a new architecture seam when the fragment stops behaving like a sibling unit at insert time

Related issues

• 2026-04-05-list-unit-fragment-proofs-should-treat-list-item-fragments-as-sibling-units-and-assert-real-paste-landings.md
• 2026-04-05-wrapper-unit-geometry-can-expand-from-paragraph-plus-nested-list-to-paragraph-plus-quote-with-the-same-sibling-unit-seam.md