Wrapper-unit geometry can expand from paragraph plus nested list to paragraph plus quote with the same sibling-unit seam

What happened

After proving list-item units with:

• one paragraph child
• one nested list child

the next honest question was whether the same sibling-unit seam also survives a different child-block mix:

• one paragraph child
• one quote child

What fixed it

It did not need a new architecture seam.

The same sibling-unit rule held:

• when the target container is a bulleted-list
• and the inserted fragment is a list-item
• treat the fragment as a sibling unit

That was enough to prove the shape through:

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

Why this works

The decisive question was not the exact child block type.

It was:

• does the wrapper unit still behave like one sibling unit at insert time?

For paragraph + quote, the answer is still yes.

The browser proof also sharpened one useful distinction:

• top-level inserts into the quote descendant rebase into the inserted sibling unit
• quote-wrapped nested-list descendant inserts can preserve the outer paragraph selection/range when that is what the real stack does

Reusable rule

When widening wrapper-unit geometry:

• change the child-block mix one sharp step at a time
• keep the sibling-unit insert seam if the fragment is still a real wrapper unit
• assert the browser/runtime landing point you actually get, not the one that feels tidier

Related issues

• 2026-04-05-list-unit-fragment-proofs-should-treat-list-item-fragments-as-sibling-units-and-assert-real-paste-landings.md