V2 fragment proofs should preserve a nested block wrapper before chasing arbitrary tree support

What happened

After top-level block and mixed-inline fragment proofs were green, the next real question was not “can we support arbitrary trees?”

It was:

• can fragment semantics survive the first nested block container at all?

The smallest honest case was a quote with paragraph children.

What fixed it

The next extension stayed narrow:

• detect sibling block containers at any depth when each child block still matches the current simple text-block proof shape
• extract the selection as wrapped fragment content when the container is nested
• insert that wrapped fragment back into the same container shape on paste

That is enough for:

• quote wrapper preserved in the fragment
• paragraph boundaries preserved in plain text and HTML transport
• quote-to-quote paste round-trip in Chromium

Why this works

The first nested-tree pressure is structural preservation, not arbitrary path math.

Top-level block proofs only have to preserve sibling blocks.

Nested quote proofs add one new requirement:

• the fragment has to remember that those sibling paragraphs belonged to a quote

Once that wrapper survives extraction and insertion, the browser proof starts measuring a real nested document shape instead of a flattened imitation.

Reusable rule

When expanding fragment semantics:

• preserve the first real nested structural wrapper before talking about arbitrary tree support

If the fragment cannot round-trip one nested block container honestly, jumping to “arbitrary trees” is just marketing.

Related issues

• 2026-04-03-slate-v2-clipboard-boundary-proof-must-split-fragment-semantics-and-dom-transport.md
• 2026-04-04-v2-mixed-inline-clipboard-proofs-should-span-top-level-blocks-before-broader-tree-shapes.md