From 79a461f79df6ce3041e2ac2741c6b3b6b35ae562 Mon Sep 17 00:00:00 2001
From: agent_coder <agent_coder@vvzvlad.xyz>
Date: Mon, 6 Jul 2026 06:26:56 +0300
Subject: [PATCH] =?UTF-8?q?test(converter):=20=D0=B2=D0=BB=D0=BE=D0=B6?=
 =?UTF-8?q?=D0=B5=D0=BD=D0=BD=D1=8B=D0=B9=20variant-A=20=D0=B3=D0=B5=D0=BD?=
 =?UTF-8?q?=D0=B5=D1=80=D0=B0=D1=82=D0=BE=D1=80=20+=20P4-=D1=84=D0=B0?=
 =?UTF-8?q?=D0=B7=D0=B7,=20=D0=B8=20=D1=84=D0=B8=D0=BA=D1=81=20=D0=B2?=
 =?UTF-8?q?=D1=81=D0=B5=D1=85=20=D0=BD=D0=B0=D0=B9=D0=B4=D0=B5=D0=BD=D0=BD?=
 =?UTF-8?q?=D1=8B=D1=85=20round-trip=20=D0=B1=D0=B0=D0=B3=D0=BE=D0=B2=20(#?=
 =?UTF-8?q?351)?=
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Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

Финальная ступень #351: генератор ЦЕЛЫХ вложенных документов (random walk
по ContentMatch схемы, min-depth fixpoint для терминирования, глубина/размер
ограничены) + инвариант P4 (фазз парсера: для ЛЮБОЙ строки markdownToProseMirror
не бросает и результат валиден по схеме). Инварианты P1 (семантический
round-trip), P2 (байтовый fixpoint со 2-го прохода), P3 (тотальность) — строгие.

Генератор сразу нашёл классы реальных багов конвертера; все починены (инварианты
НЕ ослаблялись — чинился конвертер):

- loose (много-блочные) контейнеры (listItem/taskItem/callout/detailsContent)
  склеивали блоки при реимпорте — ТИХАЯ ПОТЕРЯ ДАННЫХ; теперь blank-line
  разделитель между блок-детьми (по образцу blockquote).
- соседние sibling-списки одного marker-family (task+bullet и т.п.) сливались
  в один список с ПОТЕРЕЙ чекбокса — теперь между ними эмитится инертный
  `<!-- -->` разделитель (byte-stable round-trip).
- paragraph textAlign терялся во вложенных li/td/th.
- вложенный codeBlock терял хвостовой перевод строки.
- pageBreak/pageEmbed/subpages/transclusion дропались во вложении
  (blockquote/callout/details/li).
- медиа в columns: number→string ширины/высоты и лишний data-align (churn).
- callout `> [!type]`, вложенный в список/цитату, парсился неверно
  (prefix-aware regex).

Golden-обновления (6) — прямые следствия loose-container фикса, каждое
round-trip'ится. 2100+ сгенерированных документов (3 seed) — 0 падений P1/P2.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
---
 .../src/lib/markdown-converter.ts             | 123 +++++-
 .../src/lib/markdown-to-prosemirror.ts        | 161 ++++---
 .../src/lib/media-html.ts                     |  28 +-
 .../test/generative/doc-generator.ts          | 392 ++++++++++++++++++
 .../nested-roundtrip.property.test.ts         | 185 +++++++++
 .../test/markdown-converter-gaps.test.ts      |   4 +-
 .../test/markdown-converter.test.ts           |  47 ++-
 7 files changed, 849 insertions(+), 91 deletions(-)
 create mode 100644 packages/prosemirror-markdown/test/generative/doc-generator.ts
 create mode 100644 packages/prosemirror-markdown/test/generative/nested-roundtrip.property.test.ts

diff --git a/packages/prosemirror-markdown/src/lib/markdown-converter.ts b/packages/prosemirror-markdown/src/lib/markdown-converter.ts
index 2d173e52..481c3691 100644
--- a/packages/prosemirror-markdown/src/lib/markdown-converter.ts
+++ b/packages/prosemirror-markdown/src/lib/markdown-converter.ts
@@ -48,6 +48,52 @@ export interface ConvertProseMirrorToMarkdownOptions {
   dropResolvedCommentAnchors?: boolean;
 }
 
+/**
+ * Adjacent sibling lists that share a markdown MARKER FAMILY re-parse as ONE
+ * merged list — bulletList and taskList both emit `- ` markers (→ a single
+ * `<ul>`), and two orderedLists both emit `1.` markers (→ a single `<ol>`). The
+ * cross-type case is real data loss the editor CAN produce (e.g. a taskList
+ * followed by a bulletList: the merged `<ul>` has a mix of checkbox and plain
+ * items, so `bridgeTaskLists` refuses to convert it and every taskItem loses its
+ * checkbox). Between two such adjacent list children we emit an empty HTML comment
+ * `<!-- -->`: marked renders it as its own HTML block that interrupts the list, so
+ * the two lists stay distinct; on import the comment is inert (parseAttachedComment
+ * → null) and dropped by generateJSON, and re-export re-inserts it, so the marker
+ * is byte-stable. It fires ONLY between two adjacent same-family list nodes — no
+ * separator is emitted for any other join, so non-list output is unchanged.
+ */
+const LIST_MARKER_SEPARATOR = "<!-- -->";
+function listMarkerFamily(type: string | undefined): "ul" | "ol" | null {
+  if (type === "bulletList" || type === "taskList") return "ul";
+  if (type === "orderedList") return "ol";
+  return null;
+}
+function adjacentListsMerge(
+  prevType: string | undefined,
+  curType: string | undefined,
+): boolean {
+  const a = listMarkerFamily(prevType);
+  return a !== null && a === listMarkerFamily(curType);
+}
+/**
+ * Render each block child, inserting a `<!-- -->` separator entry between any two
+ * adjacent same-marker-family list nodes (see LIST_MARKER_SEPARATOR). Callers
+ * join the returned strings with their own context separator.
+ */
+function renderBlockChildren(
+  children: any[],
+  render: (n: any) => string,
+): string[] {
+  const out: string[] = [];
+  let prevType: string | undefined;
+  for (const child of children) {
+    if (adjacentListsMerge(prevType, child?.type)) out.push(LIST_MARKER_SEPARATOR);
+    out.push(render(child));
+    prevType = child?.type;
+  }
+  return out;
+}
+
 /**
  * Convert ProseMirror/TipTap JSON content to Markdown
  * Supports all Docmost-specific node types and extensions
@@ -347,9 +393,15 @@ export function convertProseMirrorToMarkdown(
         // lossless (the body survives) and byte-stable (it re-exports identically),
         // so it is deliberately not treated as data loss.
         const parts: string[] = [];
+        let prevDocType: string | undefined;
         for (const child of nodeContent) {
           if (child?.type === "footnotesList") continue;
+          // Keep adjacent same-family sibling lists distinct (see renderBlockChildren).
+          if (adjacentListsMerge(prevDocType, child?.type)) {
+            parts.push(LIST_MARKER_SEPARATOR);
+          }
           parts.push(processNode(child));
+          prevDocType = child?.type;
         }
         for (const [id, def] of footnoteDefs) {
           if (!referencedFootnoteIds.has(id)) {
@@ -599,20 +651,34 @@ export function convertProseMirrorToMarkdown(
         return processTaskItem(node);
 
       case "listItem":
-        return nodeContent.map(processNode).join("\n");
+        // Direct-listItem path (lists normally render via processListItem, which
+        // handles the marker + indentation). Blank line between block children so
+        // multiple paragraphs do not merge on re-parse; a `<!-- -->` entry
+        // (renderBlockChildren) separates adjacent sibling lists.
+        return renderBlockChildren(nodeContent, processNode).join("\n\n");
 
-      case "blockquote":
+      case "blockquote": {
         // Prefix EVERY line of EVERY child with "> " and separate block-level
         // children with a blank ">" line so code blocks / multi-paragraph
-        // quotes round-trip correctly.
-        return nodeContent
-          .map((n: any) =>
+        // quotes round-trip correctly. A `> <!-- -->` separator line is inserted
+        // between two adjacent same-family sibling lists (renderBlockChildren)
+        // so they stay distinct inside the quote.
+        const bqParts: string[] = [];
+        let prevBqType: string | undefined;
+        for (const n of nodeContent) {
+          if (adjacentListsMerge(prevBqType, n?.type)) {
+            bqParts.push(`> ${LIST_MARKER_SEPARATOR}`);
+          }
+          bqParts.push(
             processNode(n)
               .split("\n")
               .map((line: string) => (line.length ? `> ${line}` : ">"))
               .join("\n"),
-          )
-          .join("\n>\n");
+          );
+          prevBqType = n?.type;
+        }
+        return bqParts.join("\n>\n");
+      }
 
       case "horizontalRule":
         return "---";
@@ -787,9 +853,12 @@ export function convertProseMirrorToMarkdown(
         // blockquote-prefixed; a blank line becomes a bare `>` so the callout is
         // not split.
         const calloutType = (node.attrs?.type || "info").toLowerCase();
-        const calloutBody = nodeContent
-          .map(processNode)
-          .join("\n")
+        const calloutBody = renderBlockChildren(nodeContent, processNode)
+          // Blank line between block children (rendered as a bare `>` after the
+          // prefix pass below) so multiple paragraphs stay separate nodes instead
+          // of merging on re-parse — same rule blockquote already uses. A
+          // `<!-- -->` entry (renderBlockChildren) separates adjacent sibling lists.
+          .join("\n\n")
           .split("\n")
           .map((l: string) => (l.length ? `> ${l}` : ">"))
           .join("\n");
@@ -809,7 +878,10 @@ export function convertProseMirrorToMarkdown(
         return `<summary>${renderInlineChildren(nodeContent)}</summary>\n\n`;
 
       case "detailsContent":
-        return `${nodeContent.map(processNode).join("\n")}\n`;
+        // Blank line between block children so multiple paragraphs in a details
+        // body survive as separate nodes (a single "\n" merges them on re-parse);
+        // a `<!-- -->` entry (renderBlockChildren) separates adjacent sibling lists.
+        return `${renderBlockChildren(nodeContent, processNode).join("\n\n")}\n`;
 
       case "mathInline": {
         // #293 canon #6: inline math serializes as Obsidian-native `$LaTeX$`
@@ -1338,11 +1410,19 @@ export function convertProseMirrorToMarkdown(
         // The code itself is element TEXT content (between <code> tags), so it
         // must escape < > & — NOT the attribute escaper. The language rides in
         // a class ATTRIBUTE, so it uses escapeAttr.
+        //
+        // Read the child text RAW (as `case "codeBlock"` does) and keep it
+        // VERBATIM — do NOT strip the trailing newline. Unlike the markdown fence
+        // path (which strips then relies on marked re-adding one `\n`), the schema
+        // codeBlock parseHTML reads the `<code>` text content back byte-for-byte,
+        // so stripping here would drop a trailing newline the node legitimately
+        // carries and break the round trip inside a column/cell.
         const code = escapeHtmlText(
           children
-            .map(processNode)
-            .join("")
-            .replace(/\n+$/, ""),
+            .map((child: any) =>
+              typeof child?.text === "string" ? child.text : "",
+            )
+            .join(""),
         );
         const cls = lang ? ` class="language-${escapeAttr(lang)}"` : "";
         return `<pre><code${cls}>${code}</code></pre>`;
@@ -1484,6 +1564,13 @@ export function convertProseMirrorToMarkdown(
     const indent = " ".repeat(indentWidth);
     const lines: string[] = [];
     childStrings.forEach((child, childIndex) => {
+      // Separate consecutive block children with a BLANK line so the item is a
+      // CommonMark "loose" list item and each block stays its own node. Without
+      // it, a second paragraph (`- a\n  b`) is re-parsed as a lazy continuation
+      // of the first and the two merge into one paragraph — silent data loss.
+      // The blank line still sits INSIDE the item (the following block keeps the
+      // continuation indent), so nested lists/code blocks remain nested.
+      if (childIndex > 0) lines.push("");
       child.split("\n").forEach((line, lineIndex) => {
         if (childIndex === 0 && lineIndex === 0) {
           // First physical line of the first block gets the marker.
@@ -1500,7 +1587,9 @@ export function convertProseMirrorToMarkdown(
 
   const processListItem = (item: any, prefix: string): string => {
     const itemContent = item.content || [];
-    const childStrings = itemContent.map(processNode);
+    // A `<!-- -->` entry separates two adjacent same-family sublists inside the
+    // item so they do not merge on re-parse (see renderBlockChildren).
+    const childStrings = renderBlockChildren(itemContent, processNode);
     if (childStrings.length === 0) return prefix;
     // The rendered marker is `${prefix} ` (prefix + one space), so its width —
     // and thus the continuation indent — is prefix.length + 1. This is correct
@@ -1514,7 +1603,9 @@ export function convertProseMirrorToMarkdown(
     const checkbox = checked ? "[x]" : "[ ]";
     const prefix = `- ${checkbox}`;
     const itemContent = item.content || [];
-    const childStrings = itemContent.map(processNode);
+    // A `<!-- -->` entry separates two adjacent same-family sublists inside the
+    // item so they do not merge on re-parse (see renderBlockChildren).
+    const childStrings = renderBlockChildren(itemContent, processNode);
     // An empty task item still needs its checkbox marker; without this guard
     // the indent below produces "" and the "- [ ]"/"- [x]" row disappears.
     if (childStrings.length === 0) return prefix;
diff --git a/packages/prosemirror-markdown/src/lib/markdown-to-prosemirror.ts b/packages/prosemirror-markdown/src/lib/markdown-to-prosemirror.ts
index b5e0f2d7..f482ed5e 100644
--- a/packages/prosemirror-markdown/src/lib/markdown-to-prosemirror.ts
+++ b/packages/prosemirror-markdown/src/lib/markdown-to-prosemirror.ts
@@ -270,9 +270,10 @@ const CALLOUT_CLOSE_RE = /^:::\s*$/;
  * optional title after the type is allowed but ignored (the Docmost callout
  * schema has no title). The body is the following contiguous blockquote lines.
  */
-const CALLOUT_BQ_OPEN_RE = /^>\s*\[!(\w+)\]/;
-/** Matches any blockquote continuation line (`>` … ). */
-const BLOCKQUOTE_LINE_RE = /^>/;
+// The callout's own `>` marker may be preceded by an ENCLOSING container prefix:
+// list-item indentation (`  `) and/or blockquote markers (`> `). Group 1 captures
+// that prefix (lazily, so the LAST `>` before `[!type]` is the callout's own).
+const CALLOUT_BQ_OPEN_RE = /^([>\s]*?)>\s*\[!(\w+)\]/;
 /** Matches the start/end of a code fence (``` or ~~~), capturing the marker. */
 const CODE_FENCE_RE = /^(\s*)(`{3,}|~{3,})/;
 
@@ -402,20 +403,47 @@ async function preprocessCallouts(markdown: string): Promise<string> {
       // recurse so nested callouts (`> > [!type]`) are handled, then emit the same
       // callout div the `:::` path produces. A normal blockquote (no `[!type]` on
       // its first line) does not match and stays a blockquote.
+      //
+      // PREFIX-aware: a callout nested inside a list item and/or a blockquote is
+      // serialized with the enclosing container prefix in front of its own `>`
+      // marker — `  > [!type]` (list indent) or `> > [!type]` (blockquote). We
+      // capture that prefix, take only continuation lines carrying `prefix>`, strip
+      // it, and re-apply the prefix to the emitted HTML block so the callout div
+      // stays WITHIN its container (an unprefixed div would escape and re-parse as
+      // a top-level callout / plain blockquote — silent structure loss).
       const bqOpen = line.match(CALLOUT_BQ_OPEN_RE);
       if (bqOpen) {
-        const type = bqOpen[1].toLowerCase();
+        const prefix = bqOpen[1];
+        const type = bqOpen[2].toLowerCase();
+        const cont = prefix + ">"; // a body line = prefix + the callout's own `>`
         const bodyLines: string[] = [];
         let j = i + 1;
         for (; j < lines.length; j++) {
-          if (!BLOCKQUOTE_LINE_RE.test(lines[j])) break;
-          bodyLines.push(lines[j].replace(/^>\s?/, ""));
+          if (!lines[j].startsWith(cont)) break;
+          // Drop the prefix + `>` + one optional space, leaving the body content.
+          bodyLines.push(lines[j].slice(prefix.length).replace(/^>\s?/, ""));
         }
         const inner = await transform(bodyLines);
         const renderedInner = await markedInstance.parse(inner);
-        out.push(
-          `\n<div data-type="callout" data-callout-type="${type}">${renderedInner}</div>\n`,
-        );
+        const block = `<div data-type="callout" data-callout-type="${type}">${renderedInner}</div>`;
+        if (prefix.length === 0) {
+          // Top-level callout: blank lines isolate the HTML block.
+          out.push(`\n${block}\n`);
+        } else if (prefix.includes(">")) {
+          // Enclosing BLOCKQUOTE: prefix every line and add NO surrounding blank
+          // lines — a blank line would terminate the blockquote and split the
+          // callout out of it.
+          out.push(block.split("\n").map((l) => prefix + l).join("\n"));
+        } else {
+          // Pure LIST-ITEM indentation: re-indent and keep the blank-line
+          // separators (a loose list item), so the div sits at the marker column.
+          out.push(
+            `\n${block
+              .split("\n")
+              .map((l) => (l.length ? prefix + l : l))
+              .join("\n")}\n`,
+          );
+        }
         i = j;
         continue;
       }
@@ -597,6 +625,40 @@ function bridgeTaskLists(html: string): string {
  * (null from parseAttachedComment), an unknown name, a wrong-position comment, or
  * an unknown/empty attr value is ignored.
  */
+/**
+ * A directive comment is in ATTACHED position when it sits inside a `<p>`/`<hN>`
+ * textblock — bound to that block's text (the `attrs`/`img` conventions). Every
+ * other parent (body, document level, a block container like blockquote/details/
+ * li/column div) is STANDALONE position, where a lone-block directive
+ * (subpages/pagebreak/pageembed/transclusion) is materialized. Broadening
+ * standalone beyond body/document is what lets these nodes survive NESTED inside
+ * a blockquote/callout/details/list item (previously dropped -> silent data loss).
+ */
+function isAttachedPosition(tag: string): boolean {
+  return tag === "p" || /^h[1-6]$/.test(tag);
+}
+
+/**
+ * Place a materialized standalone-directive element in the DOM: replace the
+ * comment IN PLACE when it has a real element parent inside <body> (body itself
+ * or a nested block container), preserving document order; queue it as a leading
+ * div only when the comment is at document level (no parentElement) or directly
+ * under `<html>` (outside <body>, which `document.body.innerHTML` would drop).
+ */
+function placeStandalone(
+  comment: any,
+  el: any,
+  tag: string,
+  leadingDivs: any[],
+): void {
+  if (comment.parentElement && tag !== "html") {
+    comment.replaceWith(el);
+  } else {
+    comment.remove();
+    leadingDivs.push(el);
+  }
+}
+
 function applyCommentDirectives(html: string): string {
   // Cheap early-out: no comments at all -> nothing to intercept.
   if (!html.includes("<!--")) return html;
@@ -664,13 +726,13 @@ function applyCommentDirectives(html: string): string {
 
     if (parsed.name === "subpages" || parsed.name === "pagebreak") {
       // #293 canon #5 STANDALONE machinery. A lone comment line is rendered by
-      // marked as an HTML block; the parser places it either directly under
-      // <body> (when other content surrounds it) or at document level (when it
-      // leads the output). Both are STANDALONE position. A `subpages`/`pagebreak`
-      // comment sitting inside a `<p>`/`<hN>` (or any other element) is attached
-      // position -> INERT.
-      const standalone = tag === "" || tag === "body" || tag === "html";
-      if (!standalone) continue; // wrong position -> inert
+      // marked as its own HTML block; the parser places it under <body>, at
+      // document level (leading), or — when the directive is NESTED — inside a
+      // block CONTAINER (`<blockquote>` for blockquote/callout, `<details>`,
+      // `<li>`, a column `<div>`, …). All of those are STANDALONE position. Only a
+      // comment ATTACHED inside a `<p>`/`<hN>` (bound to that block's text) is
+      // attached position -> INERT.
+      if (isAttachedPosition(tag)) continue; // wrong position -> inert
       const div = document.createElement("div");
       if (parsed.name === "pagebreak") {
         div.setAttribute("data-type", "pageBreak");
@@ -680,26 +742,18 @@ function applyCommentDirectives(html: string): string {
           div.setAttribute("data-recursive", "true");
         }
       }
-      if (tag === "body") {
-        // In-body: replace in place so surrounding content keeps its order.
-        comment.replaceWith(div);
-      } else {
-        // Document-level (leading): drop the stray comment and queue the div to
-        // be prepended into body below.
-        comment.remove();
-        leadingDivs.push(div);
-      }
+      placeStandalone(comment, div, tag, leadingDivs);
       continue;
     }
 
     if (parsed.name === "pageembed" || parsed.name === "transclusion") {
       // #293 canon #8 STANDALONE media. Like subpages/pagebreak: a lone comment
-      // line placed under <body> or at document level (leading). An attached-
-      // position comment (inside a <p>/<hN> with a sibling) is INERT. We rebuild
-      // the schema div the raw-HTML path emits (media-html.ts) from the decoded
-      // attrs so serialize/parse stay in sync.
-      const standalone = tag === "" || tag === "body" || tag === "html";
-      if (!standalone) continue; // wrong position -> inert
+      // line placed under <body>, at document level (leading), or NESTED inside a
+      // block container (blockquote/callout/details/li/column). An ATTACHED-
+      // position comment (inside a `<p>`/`<hN>`) is INERT. We rebuild the schema
+      // div the raw-HTML path emits (media-html.ts) from the decoded attrs so
+      // serialize/parse stay in sync.
+      if (isAttachedPosition(tag)) continue; // wrong position -> inert
       const el = buildElement(
         parsed.name === "pageembed"
           ? pageEmbedToHtml({ sourcePageId: parsed.attrs.sourcePageId })
@@ -709,12 +763,7 @@ function applyCommentDirectives(html: string): string {
             }),
       );
       if (!el) continue; // defensive: builder always yields an element
-      if (tag === "body") {
-        comment.replaceWith(el);
-      } else {
-        comment.remove();
-        leadingDivs.push(el);
-      }
+      placeStandalone(comment, el, tag, leadingDivs);
       continue;
     }
 
@@ -806,18 +855,36 @@ function applyCommentDirectives(html: string): string {
 
     if (!parent) continue; // attrs comment must have an element parent
     if (parsed.name !== "attrs") continue; // unknown name -> inert
-    // #293 canon #9 ATTACHED attrs: honored only in attached position.
-    const isBlock = tag === "p" || /^h[1-6]$/.test(tag);
-    if (!isBlock) continue; // misplaced comment -> inert
     const align = parsed.attrs.textAlign;
-    if (typeof align === "string" && align) {
-      // Re-express as an inline style; the schema's textAlign parseHTML reads
-      // `el.style.textAlign` back onto the paragraph/heading node.
-      parent.style.textAlign = align;
+    // #293 canon #9 ATTACHED attrs: honored only in attached position.
+    if (tag === "p" || /^h[1-6]$/.test(tag)) {
+      // A real <p>/<hN> host (loose list item, top-level block, …): re-express as
+      // an inline style; the schema's textAlign parseHTML reads `el.style.textAlign`
+      // back onto the paragraph/heading node.
+      if (typeof align === "string" && align) parent.style.textAlign = align;
+      comment.remove();
+    } else if (tag === "li" || tag === "td" || tag === "th") {
+      // TIGHT list item / GFM table cell: marked emits the paragraph's inline
+      // content DIRECTLY inside the <li>/<td>/<th> with NO <p> wrapper, so there
+      // is no element to carry the style — generateJSON materializes the
+      // paragraph later. Wrap the host's LEADING inline content (everything up to
+      // the comment; any trailing block child such as a nested list stays put) in
+      // a <p> carrying the alignment, so the materialized paragraph re-reads it.
+      if (typeof align === "string" && align) {
+        const p = document.createElement("p");
+        p.style.textAlign = align;
+        while (parent.firstChild && parent.firstChild !== comment) {
+          p.appendChild(parent.firstChild);
+        }
+        parent.insertBefore(p, comment);
+      }
+      comment.remove();
+    } else {
+      // Misplaced `attrs` comment (not a textblock/li/cell host): inert. Consume
+      // it anyway so no attached marker ever survives into the parsed body
+      // (matches the pre-existing "consume regardless" behaviour).
+      comment.remove();
     }
-    // Consume the marker regardless (unknown keys are simply ignored) so no
-    // attached comment ever survives into the parsed body.
-    comment.remove();
   }
   // Prepend any document-level (leading) standalone divs into body, preserving
   // their document order relative to each other and ahead of existing content.
diff --git a/packages/prosemirror-markdown/src/lib/media-html.ts b/packages/prosemirror-markdown/src/lib/media-html.ts
index 733f5aed..a924fe92 100644
--- a/packages/prosemirror-markdown/src/lib/media-html.ts
+++ b/packages/prosemirror-markdown/src/lib/media-html.ts
@@ -46,7 +46,11 @@ export function videoToHtml(attrs: Record<string, any>): string {
   if (attrs.width != null) parts.push(`width="${escapeAttr(attrs.width)}"`);
   if (attrs.height != null) parts.push(`height="${escapeAttr(attrs.height)}"`);
   if (attrs.size != null) parts.push(`data-size="${escapeAttr(attrs.size)}"`);
-  if (attrs.align) parts.push(`data-align="${escapeAttr(attrs.align)}"`);
+  // align default is "center" (schema): OMIT it so a bare/center node stays
+  // clean and parse's re-materialized "center" default is not a P2 churn — only
+  // a genuinely non-default left/right emits data-align (mirrors imageToHtml).
+  if (attrs.align && attrs.align !== "center")
+    parts.push(`data-align="${escapeAttr(attrs.align)}"`);
   if (attrs.aspectRatio != null)
     parts.push(`data-aspect-ratio="${escapeAttr(attrs.aspectRatio)}"`);
   return `<div><video ${parts.join(" ")}></video></div>`;
@@ -62,7 +66,9 @@ export function youtubeToHtml(attrs: Record<string, any>): string {
     parts.push(`data-width="${escapeAttr(attrs.width)}"`);
   if (attrs.height != null)
     parts.push(`data-height="${escapeAttr(attrs.height)}"`);
-  if (attrs.align) parts.push(`data-align="${escapeAttr(attrs.align)}"`);
+  // "center" is the schema default -> omit (see videoToHtml rationale).
+  if (attrs.align && attrs.align !== "center")
+    parts.push(`data-align="${escapeAttr(attrs.align)}"`);
   return `<div ${parts.join(" ")}></div>`;
 }
 
@@ -97,7 +103,9 @@ export function diagramToHtml(
   if (attrs.size != null) parts.push(`data-size="${escapeAttr(attrs.size)}"`);
   if (attrs.aspectRatio != null)
     parts.push(`data-aspect-ratio="${escapeAttr(attrs.aspectRatio)}"`);
-  if (attrs.align) parts.push(`data-align="${escapeAttr(attrs.align)}"`);
+  // "center" is the schema default -> omit (see videoToHtml rationale).
+  if (attrs.align && attrs.align !== "center")
+    parts.push(`data-align="${escapeAttr(attrs.align)}"`);
   if (attrs.attachmentId)
     parts.push(`data-attachment-id="${escapeAttr(attrs.attachmentId)}"`);
   return `<div ${parts.join(" ")}></div>`;
@@ -110,10 +118,18 @@ export function embedToHtml(attrs: Record<string, any>): string {
     `data-src="${escapeAttr(attrs.src ?? "")}"`,
     `data-provider="${escapeAttr(attrs.provider ?? "")}"`,
   ];
-  if (attrs.align) parts.push(`data-align="${escapeAttr(attrs.align)}"`);
-  if (attrs.width != null)
+  // "center" is the schema default -> omit (see videoToHtml rationale).
+  if (attrs.align && attrs.align !== "center")
+    parts.push(`data-align="${escapeAttr(attrs.align)}"`);
+  // embed width/height default to the NUMBERS 800/600 (schema). Getting a data-
+  // attribute back always yields a STRING, so emitting the default here would
+  // round-trip 800 -> "800" (a number->string P1 divergence canonicalize does
+  // NOT normalize). OMIT the defaults so parse re-materializes the numeric
+  // default instead — mirrors the top-level embed path (markdown-converter.ts),
+  // which also emits width/height only when they differ from 800/600.
+  if (attrs.width != null && attrs.width !== 800)
     parts.push(`data-width="${escapeAttr(attrs.width)}"`);
-  if (attrs.height != null)
+  if (attrs.height != null && attrs.height !== 600)
     parts.push(`data-height="${escapeAttr(attrs.height)}"`);
   return `<div ${parts.join(" ")}></div>`;
 }
diff --git a/packages/prosemirror-markdown/test/generative/doc-generator.ts b/packages/prosemirror-markdown/test/generative/doc-generator.ts
new file mode 100644
index 00000000..90c82dca
--- /dev/null
+++ b/packages/prosemirror-markdown/test/generative/doc-generator.ts
@@ -0,0 +1,392 @@
+/**
+ * Nested whole-document generator (#351, PR 2) — "variant A": a random walk over
+ * the schema's ContentMatch automaton.
+ *
+ * Where the FLAT generator (node-generators.ts) emits `{doc:[ <one target> ]}`,
+ * this module produces arbitrarily DEEP, valid ProseMirror documents. Validity is
+ * NOT hand-asserted: it comes straight from the schema. To fill a container's
+ * block content we start at `nodeType.contentMatch` and walk the automaton —
+ * enumerate the legal next node types (`match.edge(i).type`), let fast-check pick
+ * one (or STOP once `match.validEnd`), generate that child RECURSIVELY, then
+ * advance the automaton via `match.matchType(childType)`. A bad walk therefore
+ * cannot emit a structurally-invalid doc (the `generator validity` test guards
+ * this with `schema.nodeFromJSON(json).check()`).
+ *
+ * ── What the walk drives, and what it delegates ──────────────────────────────
+ * The walk owns BLOCK STRUCTURE (which blocks nest inside which containers, in
+ * what order and how deep). Two things it deliberately delegates, to avoid
+ * REGRESSING the byte-stable space the flat suite already proved empirically:
+ *
+ *   - INLINE content of textblocks (paragraph/heading/codeBlock/detailsSummary)
+ *     is filled from text-arbitraries.ts — the exact hostile-but-byte-stable
+ *     inline corpus the flat suite established. Walking the inline ContentMatch
+ *     instead would re-derive (and re-fail) the text-space limitations the flat
+ *     suite already pins, which is not this generator's job.
+ *   - Node ATTRS come from nodeAttrsArb(type, 'p1') — the round-trip-safe
+ *     attribute space. Attribute-degenerate fuzzing (P2/P3 over 'fuzz') is the
+ *     flat suite's concern; the nested suite isolates STRUCTURAL round-trip, so
+ *     it stays in 'p1' and does not re-litigate frozen/pinned attributes.
+ *
+ * ── Coordination the automaton cannot express ────────────────────────────────
+ * A ContentMatch guarantees a child SEQUENCE is legal, but not cross-sibling
+ * invariants. Two nodes need coordination the walk injects by hand:
+ *   - `table`: GFM needs a RECTANGULAR grid with column-consistent alignment.
+ *     The automaton happily allows ragged rows / per-cell align, which are not a
+ *     converter bug — just a malformed table. So a table is generated ATOMICALLY
+ *     (same shape the flat suite proved) rather than walked.
+ *   - `columns`: the `layout` attr must agree with the column COUNT. We pick the
+ *     layout, derive the count, then walk each column's block body normally — so
+ *     columns still gain real nested content, only the count is coordinated.
+ *
+ * ── Excluded from the nested walk ────────────────────────────────────────────
+ * Footnote nodes (footnoteReference / footnotesList / footnoteDefinition) need a
+ * DOCUMENT-GLOBAL id match between a reference and its definition. That
+ * coordination is owned by the flat suite's `footnotes` generator; placing them
+ * independently here would fabricate id mismatches that look like converter bugs
+ * but are generator defects. They are filtered out of the walk (documented in
+ * EXCLUDED). The completeness contract lives in the FLAT suite and is unaffected.
+ *
+ * ── Termination / budgets ────────────────────────────────────────────────────
+ * Two bounds keep every doc finite and the suite fast:
+ *   - MAX_DEPTH — a hard cap on block-nesting depth. A precomputed `minDepth`
+ *     fixpoint (the minimum extra nesting a subtree of each type needs to be
+ *     valid) lets the walk pick a CONTAINER child only when there is depth
+ *     headroom to complete it — so the walk can never paint itself into a corner
+ *     where a required child cannot fit (no invalid docs, guaranteed termination).
+ *   - NODE_BUDGET — a soft cap on total nodes; as it runs low the walk biases
+ *     toward STOP (when validEnd) or toward cheap terminating children.
+ */
+import fc from 'fast-check';
+import { getSchema } from '@tiptap/core';
+import { docmostExtensions } from '../../src/lib/docmost-schema.js';
+import { nodeAttrsArb } from './attr-arbitraries.js';
+import {
+  inlineContentArb,
+  headingInlineContentArb,
+  plainInlineContentArb,
+  phraseArb,
+} from './text-arbitraries.js';
+
+/** The exact ProseMirror schema the converter targets (built per the issue). */
+export const schema = getSchema(docmostExtensions as never);
+
+/** Hard cap on block-nesting depth (doc = depth 0). Kept in the issue's 4–5 band. */
+export const MAX_DEPTH = 4;
+/**
+ * Soft cap on total node count per generated document. Kept moderate: every P1/P2
+ * run parses the emitted markdown through jsdom (heavy), so 100+ node docs across
+ * hundreds of runs exhaust the worker heap. 60 still yields deeply-nested docs
+ * (depth 4) while keeping the suite within memory.
+ */
+export const NODE_BUDGET = 60;
+
+/**
+ * Nodes kept OUT of the nested walk: footnote nodes need a doc-global id match a
+ * local walk cannot coordinate (owned by the flat suite's `footnotes` generator).
+ */
+const EXCLUDED = new Set<string>([
+  'footnoteReference',
+  'footnotesList',
+  'footnoteDefinition',
+]);
+
+/**
+ * Structural-only children that carry `group: "block"` in the schema and so leak
+ * into EVERY block container's ContentMatch, even though the editor only ever
+ * places them inside their one true parent. Choosing them freely (e.g. a bare
+ * `column` at the document root) fabricates documents no editor produces and that
+ * the converter is not designed to round-trip — a GENERATOR artifact, not a
+ * converter bug. They are admitted ONLY when the container being filled is their
+ * dedicated parent. (`column` is in fact always built inside columnsArb, so this
+ * just double-guards it.)
+ */
+const DEDICATED_PARENT: Record<string, string> = {
+  column: 'columns',
+  detailsSummary: 'details',
+  detailsContent: 'details',
+};
+
+/** Is child type `t` legal as a freely-chosen child of container `parentType`? */
+function childAllowedUnder(t: string, parentType: string): boolean {
+  const dedicated = DEDICATED_PARENT[t];
+  return dedicated === undefined || dedicated === parentType;
+}
+
+/** Textblock (inlineContent) types — filled from the proven inline corpus. */
+function isTextblock(typeName: string): boolean {
+  return !!schema.nodes[typeName]?.isTextblock;
+}
+/** Leaf/atom types — no content, only generated attrs. */
+function isLeaf(typeName: string): boolean {
+  return !!schema.nodes[typeName]?.isLeaf;
+}
+
+// ---------------------------------------------------------------------------
+// minDepth fixpoint: the minimum EXTRA block-nesting depth a valid subtree
+// rooted at each node type requires. Leaves and textblocks need 0 (a textblock
+// is satisfied by inline content, no block recursion). A container needs
+// 1 + the cheapest way to satisfy its ContentMatch. Computed as a min–max path
+// to `validEnd` over the automaton, iterated to a fixpoint over node types.
+// ---------------------------------------------------------------------------
+
+/** Cheapest (min over reachable validEnd of max child minDepth) to complete a match. */
+function minCompletion(
+  match: any,
+  md: Record<string, number>,
+  seen: Set<any>,
+  parentType: string,
+): number {
+  let best = match.validEnd ? 0 : Infinity;
+  if (seen.has(match)) return best; // a cycle never completes more cheaply
+  seen.add(match);
+  for (let i = 0; i < match.edgeCount; i++) {
+    const edge = match.edge(i);
+    const t = edge.type.name;
+    if (EXCLUDED.has(t) || t === 'text') continue;
+    if (!childAllowedUnder(t, parentType)) continue;
+    const childCost = md[t];
+    if (childCost === undefined || childCost === Infinity) continue;
+    const rest = minCompletion(edge.next, md, seen, parentType);
+    if (rest === Infinity) continue;
+    best = Math.min(best, Math.max(childCost, rest));
+  }
+  seen.delete(match);
+  return best;
+}
+
+function computeMinDepth(): Record<string, number> {
+  const md: Record<string, number> = {};
+  for (const name of Object.keys(schema.nodes)) {
+    md[name] = isLeaf(name) || isTextblock(name) ? 0 : Infinity;
+  }
+  let changed = true;
+  while (changed) {
+    changed = false;
+    for (const name of Object.keys(schema.nodes)) {
+      if (md[name] === 0) continue; // leaves/textblocks fixed at 0
+      const nt: any = schema.nodes[name];
+      const completion = minCompletion(nt.contentMatch, md, new Set(), name);
+      const next = completion === Infinity ? Infinity : 1 + completion;
+      if (next < md[name]) {
+        md[name] = next;
+        changed = true;
+      }
+    }
+  }
+  return md;
+}
+
+const MIN_DEPTH = computeMinDepth();
+
+// ---------------------------------------------------------------------------
+// Leaf / textblock builders (attrs from 'p1', inline from the proven corpus).
+// ---------------------------------------------------------------------------
+
+function attachAttrs(typeName: string, base: Record<string, unknown> = {}) {
+  return nodeAttrsArb(typeName, 'p1', base).map((attrs) => {
+    const node: any = { type: typeName };
+    if (Object.keys(attrs).length) node.attrs = attrs;
+    return node;
+  });
+}
+
+/** A leaf/atom block: attrs only, no content. */
+function leafArb(typeName: string): fc.Arbitrary<any> {
+  return attachAttrs(typeName);
+}
+
+/** A textblock, inline content taken from the byte-stable flat corpus. */
+function textblockArb(typeName: string): fc.Arbitrary<any> {
+  if (typeName === 'codeBlock') {
+    return fc
+      .tuple(
+        nodeAttrsArb('codeBlock', 'p1'),
+        // Fenced code re-imports with a TRAILING NEWLINE (flat suite finding);
+        // author it so the doc is already at the round-trip fixpoint.
+        fc.array(phraseArb, { minLength: 1, maxLength: 3 }).map((l) => l.join('\n') + '\n'),
+      )
+      .map(([attrs, code]) => ({
+        type: 'codeBlock',
+        ...(Object.keys(attrs).length ? { attrs } : {}),
+        content: [{ type: 'text', text: code }],
+      }));
+  }
+  const inline =
+    typeName === 'heading'
+      ? headingInlineContentArb
+      : typeName === 'detailsSummary'
+        ? plainInlineContentArb
+        : inlineContentArb;
+  return fc
+    .tuple(nodeAttrsArb(typeName, 'p1'), inline)
+    .map(([attrs, content]) => ({
+      type: typeName,
+      ...(Object.keys(attrs).length ? { attrs } : {}),
+      content,
+    }));
+}
+
+// ---------------------------------------------------------------------------
+// Coordinated builders: table (atomic, rectangular, column-consistent align)
+// and columns (layout coupled to count, bodies walked).
+// ---------------------------------------------------------------------------
+
+/** A rectangular GFM-safe table (mirrors the flat suite's proven shape). */
+function tableArb(): fc.Arbitrary<any> {
+  return fc.integer({ min: 1, max: 3 }).chain((cols) => {
+    // One alignment per COLUMN, identical on header + every body cell, so the
+    // second export cannot re-align and churn.
+    const alignsArb = fc.array(fc.constantFrom(undefined, 'left', 'center', 'right'), {
+      minLength: cols,
+      maxLength: cols,
+    });
+    const cell = (header: boolean, align?: string) =>
+      phraseArb.map((t) => ({
+        type: header ? 'tableHeader' : 'tableCell',
+        attrs: { colspan: 1, rowspan: 1, ...(align ? { align } : {}) },
+        content: [{ type: 'paragraph', content: [{ type: 'text', text: t }] }],
+      }));
+    return alignsArb.chain((aligns) => {
+      const headerRow = fc
+        .tuple(...aligns.map((a) => cell(true, a)))
+        .map((cells) => ({ type: 'tableRow', content: cells }));
+      const bodyRow = fc
+        .tuple(...aligns.map((a) => cell(false, a)))
+        .map((cells) => ({ type: 'tableRow', content: cells }));
+      return fc
+        .tuple(headerRow, fc.array(bodyRow, { minLength: 1, maxLength: 2 }))
+        .map(([h, body]) => ({ type: 'table', content: [h, ...body] }));
+    });
+  });
+}
+
+/** A columns block: layout ↔ count coupled, each column body walked as blocks. */
+function columnsArb(depth: number, budget: number): fc.Arbitrary<any> {
+  return fc
+    .constantFrom('two_equal', 'three_equal', 'left_sidebar', 'right_sidebar')
+    .chain((layout) => {
+      const count = layout === 'three_equal' ? 3 : 2;
+      const columnType: any = schema.nodes.column;
+      // Split the remaining budget across the fixed number of columns.
+      const per = Math.max(2, Math.floor((budget - 1) / count));
+      return nodeAttrsArb('columns', 'p1', { layout, widthMode: 'normal' }).chain((attrs) =>
+        fc
+          .tuple(
+            ...Array.from({ length: count }, () =>
+              fillMatch(columnType.contentMatch, depth + 1, per, 'column').map(({ children }) => ({
+                type: 'column',
+                content: children,
+              })),
+            ),
+          )
+          .map((cols) => ({ type: 'columns', attrs, content: cols })),
+      );
+    });
+}
+
+// ---------------------------------------------------------------------------
+// The ContentMatch walk.
+// ---------------------------------------------------------------------------
+
+/** Count every node in a subtree (block + inline), for budget accounting. */
+function countNodes(node: any): number {
+  let n = 1;
+  for (const c of node.content ?? []) n += countNodes(c);
+  return n;
+}
+
+/** Build a single child node of a given type at `depth`, within `budget`. */
+function blockNode(typeName: string, depth: number, budget: number): fc.Arbitrary<any> {
+  if (typeName === 'table') return tableArb();
+  if (typeName === 'columns') return columnsArb(depth, budget);
+  if (isTextblock(typeName)) return textblockArb(typeName);
+  if (isLeaf(typeName)) return leafArb(typeName);
+  // Generic container: attrs from 'p1', block content from the automaton walk.
+  const nt: any = schema.nodes[typeName];
+  return nodeAttrsArb(typeName, 'p1').chain((attrs) =>
+    fillMatch(nt.contentMatch, depth, budget - 1, typeName).map(({ children }) => {
+      const node: any = { type: typeName };
+      if (Object.keys(attrs).length) node.attrs = attrs;
+      if (children.length) node.content = children;
+      return node;
+    }),
+  );
+}
+
+/**
+ * Fill a container's block content by walking its ContentMatch automaton from
+ * `match`. Returns the children array plus the budget left after them.
+ */
+function fillMatch(
+  match: any,
+  depth: number,
+  budget: number,
+  parentType: string,
+): fc.Arbitrary<{ children: any[]; budget: number }> {
+  const canStop = match.validEnd;
+  // A child lives at depth+1; only pick it if its subtree can complete within
+  // MAX_DEPTH. This headroom rule is what makes the walk deadlock-free.
+  const headroom = MAX_DEPTH - (depth + 1);
+  const edges: { t: string; next: any }[] = [];
+  if (headroom >= 0) {
+    for (let i = 0; i < match.edgeCount; i++) {
+      const edge = match.edge(i);
+      const t = edge.type.name;
+      if (EXCLUDED.has(t) || t === 'text') continue;
+      if (!childAllowedUnder(t, parentType)) continue;
+      if ((MIN_DEPTH[t] ?? Infinity) > headroom) continue;
+      edges.push({ t, next: edge.next });
+    }
+  }
+
+  // Decide the next action: STOP (if allowed) or extend with one more child.
+  // Bias toward stopping when the budget is spent; force a child only when the
+  // match is not yet at a valid end.
+  const pool: { weight: number; arbitrary: fc.Arbitrary<{ t: string; next: any } | null> }[] = [];
+  const canGo = edges.length > 0 && (budget > 0 || !canStop);
+  if (canStop) {
+    // Stop is weighted higher when the budget is low so docs stay bounded.
+    pool.push({ weight: budget > 0 ? 2 : 5, arbitrary: fc.constant(null) });
+  }
+  if (canGo && !(canStop && budget <= 0)) {
+    pool.push({ weight: 3, arbitrary: fc.constantFrom(...edges) });
+  }
+  // Forced continuation: not a valid end yet and (budget exhausted) — must place
+  // a mandatory child regardless of budget.
+  if (pool.length === 0) {
+    if (edges.length > 0) {
+      pool.push({ weight: 1, arbitrary: fc.constantFrom(...edges) });
+    } else {
+      // No legal child and not required to place one: stop with what we have.
+      return fc.constant({ children: [], budget });
+    }
+  }
+
+  return fc.oneof(...pool).chain((choice) => {
+    if (choice === null) return fc.constant({ children: [], budget });
+    return blockNode(choice.t, depth + 1, budget).chain((node) => {
+      const cost = countNodes(node);
+      return fillMatch(choice.next, depth, budget - cost, parentType).map(
+        ({ children, budget: left }) => ({
+          children: [node, ...children],
+          budget: left,
+        }),
+      );
+    });
+  });
+}
+
+/**
+ * The nested-document arbitrary: a valid, arbitrarily-deep ProseMirror doc built
+ * by walking the schema from the document root. Attrs stay in the round-trip-safe
+ * 'p1' space; inline content reuses the byte-stable flat corpus.
+ */
+export const docArb: fc.Arbitrary<any> = fillMatch(
+  schema.nodes.doc.contentMatch,
+  0,
+  NODE_BUDGET,
+  'doc',
+).map(({ children }) => ({ type: 'doc', content: children }));
+
+/** The precomputed minDepth table, exported for inspection/debugging. */
+export { MIN_DEPTH };
diff --git a/packages/prosemirror-markdown/test/generative/nested-roundtrip.property.test.ts b/packages/prosemirror-markdown/test/generative/nested-roundtrip.property.test.ts
new file mode 100644
index 00000000..a5c773c0
--- /dev/null
+++ b/packages/prosemirror-markdown/test/generative/nested-roundtrip.property.test.ts
@@ -0,0 +1,185 @@
+import { describe, expect, it, vi } from 'vitest';
+import fc from 'fast-check';
+// Real converters. Importing markdownToProseMirror (transitively, via index)
+// mutates the global DOM via jsdom at module load — expected, required for
+// @tiptap/html's generateJSON under Node (same as the flat sibling suite).
+import {
+  convertProseMirrorToMarkdown,
+  markdownToProseMirror,
+  docsCanonicallyEqual,
+  canonicalizeContent,
+} from '../../src/lib/index.js';
+import { firstDivergence } from '../roundtrip-helpers.js';
+import { schema, docArb } from './doc-generator.js';
+
+// Each run does a real convert + jsdom parse; give ample headroom so the suite
+// is deterministic under parallel worker load (matching the flat sibling suite).
+vi.setConfig({ testTimeout: 60000 });
+
+// ---------------------------------------------------------------------------
+// #351 PR 2 — GENERATIVE round-trip over NESTED (whole-document) docs produced
+// by the ContentMatch random walk (doc-generator.ts). The invariants mirror the
+// flat suite, plus a parser-fuzz totality property (P4):
+//
+//   P1 — semantic round-trip: docsCanonicallyEqual(mdToPm(pmToMd(d)), d)
+//   P2 — byte fixpoint (2nd pass): pmToMd(mdToPm(pmToMd(d))) === pmToMd(d)
+//        (the FIRST pass may normalize once; the SECOND pass must be a fixpoint)
+//   P3 — totality: neither converter throws; bounded.
+//   P4 — parser fuzz totality: for ANY string, markdownToProseMirror does NOT
+//        throw and returns a SCHEMA-VALID document.
+//
+// GUARDRAIL: a P1/P2/P3/P4 failure means the generator FOUND A REAL CONVERTER
+// BUG. These invariants are kept STRICT — no it.fails / skip / weakening. A
+// failure prints the shrunk minimal counterexample for triage.
+// ---------------------------------------------------------------------------
+
+const SEED = 20250705;
+// The nested walk builds far heavier docs than the flat suite (each P1/P2 run
+// parses the emitted markdown through jsdom), so keep the run count moderate to
+// hold runtime and worker memory in budget while still exercising deep
+// structures. P4 (cheap string parsing) runs at a higher count below.
+const NUM_RUNS = 100;
+
+const pmToMd = (doc: unknown): string => convertProseMirrorToMarkdown(doc);
+const mdToPm = (md: string): Promise<any> => markdownToProseMirror(md);
+
+async function roundTrip(doc: unknown): Promise<{ md1: string; md2: string; doc2: any }> {
+  const md1 = pmToMd(doc);
+  const doc2 = await mdToPm(md1);
+  const md2 = pmToMd(doc2);
+  return { md1, md2, doc2 };
+}
+
+describe('#351 nested generative round-trip — generator validity', () => {
+  it('every generated nested doc passes schema.nodeFromJSON(...).check()', () => {
+    // A nested generator that emits an invalid ProseMirror document is a
+    // GENERATOR bug — the ContentMatch walk must only produce schema-valid docs.
+    fc.assert(
+      fc.property(docArb, (doc) => {
+        schema.nodeFromJSON(doc).check(); // throws on an invalid doc
+        return true;
+      }),
+      { numRuns: NUM_RUNS * 2, seed: SEED },
+    );
+  });
+});
+
+// ── STATUS: P1/P2/P3/P4 all GREEN. The nested generator originally surfaced a
+// batch of real converter bugs; all were fixed in the serializer/parser (see the
+// #351 hand-off). For the record, the classes it found and that are now fixed:
+//   • Loose (multi-block) list items / task items / callouts / details bodies were
+//     joined with a single "\n", so every block after the first merged into the
+//     first paragraph on re-parse (silent content loss) — now blank-line separated.
+//   • Paragraph `textAlign` was dropped inside a TIGHT list item (no <p> host).
+//   • A nested codeBlock lost its trailing newline on the raw-HTML path.
+//   • Media (embed/video/youtube/drawio/excalidraw) inside `columns` churned a
+//     default `data-align` and coerced embed's numeric width/height to strings.
+//   • pageBreak / pageEmbed / subpages / transclusion were dropped when nested in
+//     blockquote / callout / details / list item (standalone-comment position).
+//   • Callouts nested in a list item or a blockquote (`  > [!type]` / `> > [!type]`)
+//     were re-parsed as plain blockquotes (prefix-unaware callout preprocessor).
+//   • Two adjacent sibling lists sharing a marker family (bulletList/taskList →
+//     `<ul>`; orderedList → `<ol>`) merged into one list on re-parse — and for the
+//     cross-type case (taskList beside bulletList) the merged `<ul>` LOST every
+//     taskItem checkbox. The serializer now emits a `<!-- -->` separator between
+//     such adjacent lists (markdown-converter.ts renderBlockChildren), so they stay
+//     distinct and round-trip; the generator therefore emits them freely again.
+describe('#351 nested generative round-trip — properties', () => {
+  it('P1 — semantic round-trip: docsCanonicallyEqual(mdToPm(pmToMd(d)), d)', async () => {
+    await fc.assert(
+      fc.asyncProperty(docArb, async (doc) => {
+        const { doc2 } = await roundTrip(doc);
+        if (!docsCanonicallyEqual(doc2, doc)) {
+          const div = firstDivergence(
+            JSON.parse(JSON.stringify(canonicalizeContent(doc2))),
+            JSON.parse(JSON.stringify(canonicalizeContent(doc))),
+          );
+          throw new Error(
+            `P1 divergence @ ${div?.path}: got=${JSON.stringify(div?.a)} want=${JSON.stringify(div?.b)}`,
+          );
+        }
+      }),
+      { numRuns: NUM_RUNS, seed: SEED },
+    );
+  });
+
+  it('P2 — byte fixpoint: pmToMd(mdToPm(pmToMd(d))) === pmToMd(d)', async () => {
+    await fc.assert(
+      fc.asyncProperty(docArb, async (doc) => {
+        const { md1, md2 } = await roundTrip(doc);
+        expect(md2).toBe(md1);
+      }),
+      { numRuns: NUM_RUNS, seed: SEED },
+    );
+  });
+
+  it('P3 — totality: neither converter throws', async () => {
+    await fc.assert(
+      fc.asyncProperty(docArb, async (doc) => {
+        await roundTrip(doc);
+      }),
+      { numRuns: NUM_RUNS, seed: SEED },
+    );
+  });
+});
+
+// ---------------------------------------------------------------------------
+// P4 — parser fuzz. Independent of the doc generator: for ANY input string the
+// PARSER (markdownToProseMirror) must be TOTAL — never throw — and must always
+// return a schema-valid document. The corpus mixes raw unicode strings with
+// strings assembled from markdown-significant fragments (headings, list bullets,
+// fences, pipes, thematic breaks, HTML-ish snippets) to probe the block/inline
+// parsers on hostile but plausible input.
+// ---------------------------------------------------------------------------
+
+const mdFragmentArb: fc.Arbitrary<string> = fc.constantFrom(
+  '# ', '## ', '### ###', '- ', '* ', '+ ', '1. ', '> ', '>> ',
+  '```', '```js', '~~~', '---', '***', '___', '| a | b |', '|---|---|',
+  '[link](http://x)', '![img](http://x)', '**', '__', '~~', '`code`',
+  '<div>', '</div>', '<b>', '<!-- c -->', '<table>', '<br>', '&amp;',
+  '\t', '\n', '  ', '\\', '^[fn]', '[^1]:', '- [ ] ', '- [x] ',
+  '$$', '$x$', ':::', '{.class}', '\u0000', '\uFEFF', '😀', 'مرحبا',
+);
+
+// Full-unicode strings (fast-check v4 replaced fullUnicodeString with the
+// `unit: 'binary'` string option, which draws over the whole code-point range).
+const fullUnicodeStringArb = (max?: number) =>
+  fc.string({ unit: 'binary', ...(max !== undefined ? { maxLength: max } : {}) });
+
+const assembledMarkdownArb: fc.Arbitrary<string> = fc
+  .array(fc.oneof(mdFragmentArb, fc.string(), fullUnicodeStringArb(8)), {
+    minLength: 1,
+    maxLength: 12,
+  })
+  .map((parts) => parts.join(''));
+
+const parserInputArb: fc.Arbitrary<string> = fc.oneof(
+  { weight: 2, arbitrary: fc.string() },
+  { weight: 2, arbitrary: fullUnicodeStringArb() },
+  { weight: 3, arbitrary: assembledMarkdownArb },
+  { weight: 1, arbitrary: fc.array(mdFragmentArb, { minLength: 1, maxLength: 8 }).map((p) => p.join('\n')) },
+);
+
+describe('#351 parser fuzz — totality on arbitrary input (P4)', () => {
+  it('P4 — markdownToProseMirror never throws and always returns a schema-valid doc', async () => {
+    await fc.assert(
+      fc.asyncProperty(parserInputArb, async (s) => {
+        let result: any;
+        try {
+          result = await mdToPm(s);
+        } catch (e: any) {
+          throw new Error(`P4 parser THREW on input ${JSON.stringify(s)}: ${e?.message ?? e}`);
+        }
+        try {
+          schema.nodeFromJSON(result).check();
+        } catch (e: any) {
+          throw new Error(
+            `P4 parser produced an INVALID doc for input ${JSON.stringify(s)}: ${e?.message ?? e}\n` +
+              `doc=${JSON.stringify(result).slice(0, 600)}`,
+          );
+        }
+      }),
+      { numRuns: NUM_RUNS * 2, seed: SEED },
+    );
+  });
+});
diff --git a/packages/prosemirror-markdown/test/markdown-converter-gaps.test.ts b/packages/prosemirror-markdown/test/markdown-converter-gaps.test.ts
index 4aae6b3b..4f864fea 100644
--- a/packages/prosemirror-markdown/test/markdown-converter-gaps.test.ts
+++ b/packages/prosemirror-markdown/test/markdown-converter-gaps.test.ts
@@ -374,7 +374,9 @@ describe('converter gap coverage — emission branches (specs 1–11)', () => {
         ],
       }),
     );
-    expect(out).toBe('- [ ] top\n  - child');
+    // Block children of a task item are blank-line separated (loose list) per the
+    // #351 fix; the sublist stays at the fixed 2-column continuation indent.
+    expect(out).toBe('- [ ] top\n\n  - child');
   });
 
   // 10. A bulletList inside a blockquote: each list line independently prefixed.
diff --git a/packages/prosemirror-markdown/test/markdown-converter.test.ts b/packages/prosemirror-markdown/test/markdown-converter.test.ts
index 2a39de6c..bd4e734b 100644
--- a/packages/prosemirror-markdown/test/markdown-converter.test.ts
+++ b/packages/prosemirror-markdown/test/markdown-converter.test.ts
@@ -198,7 +198,11 @@ describe('convertProseMirrorToMarkdown', () => {
         }),
       );
       // First line carries the marker; the nested list is indented 2 columns.
-      expect(out).toBe('- parent\n  - child');
+      // Block children of a list item are separated by a BLANK line (loose list):
+      // this is the #351 fix — a single "\n" let a following block merge into the
+      // first paragraph on re-parse (silent content loss). The blank line stays
+      // inside the item, so the sublist remains nested at the 2-col marker column.
+      expect(out).toBe('- parent\n\n  - child');
     });
 
     it('nested ordered list indents by the wider 3-col marker width', () => {
@@ -219,8 +223,9 @@ describe('convertProseMirrorToMarkdown', () => {
           ],
         }),
       );
-      // "1. " is 3 columns wide, so the continuation indent is 3 spaces.
-      expect(out).toBe('1. parent\n   1. child');
+      // "1. " is 3 columns wide, so the continuation indent is 3 spaces. Block
+      // children are blank-line separated (loose list) per the #351 fix.
+      expect(out).toBe('1. parent\n\n   1. child');
     });
   });
 
@@ -539,11 +544,12 @@ describe('convertProseMirrorToMarkdown', () => {
       );
 
       // The 10th marker is the 4-column "10. "; the nested sublist line must be
-      // indented exactly 4 spaces (prefix.length 3 + 1), NOT 3.
-      expect(out).toContain('10. j\n    1. x');
+      // indented exactly 4 spaces (prefix.length 3 + 1), NOT 3. Block children are
+      // blank-line separated (loose list) per the #351 fix.
+      expect(out).toContain('10. j\n\n    1. x');
       // Guard against the off-by-one (3-space) regression that would re-parse
       // the sublist as loose/sibling content on import.
-      expect(out).not.toContain('10. j\n   1. x');
+      expect(out).not.toContain('10. j\n\n   1. x');
       // And the single-digit items keep the narrower 3-column marker (no body
       // continuation here, but the marker itself must stay "1. ".."9. ").
       expect(out.startsWith('1. a\n2. b\n')).toBe(true);
@@ -580,17 +586,18 @@ describe('convertProseMirrorToMarkdown', () => {
           content: [para(text('line1')), para(text('line2'))],
         }),
       );
-      // NOTE(review): the spec predicted ':::warning\nline1\n\nline2\n:::' (a
-      // The converter joins the callout's rendered children with a single '\n'
-      // and emits an Obsidian-native callout: a `> [!type]` opener plus one
-      // `>`-prefixed body line per content line. We pin the lowercasing
-      // (WARNING -> warning) and the multi-child join.
-      expect(out).toBe('> [!warning]\n> line1\n> line2');
+      // The converter emits an Obsidian-native callout: a `> [!type]` opener plus
+      // one `>`-prefixed body line per content line. Block children are separated
+      // by a blank `>` line (#351 fix): a single '\n' let the two paragraphs merge
+      // into one on re-parse. We pin the lowercasing (WARNING -> warning) and the
+      // blank-line-separated multi-child join.
+      expect(out).toBe('> [!warning]\n> line1\n>\n> line2');
       // The type is lowercased (an uppercase `[!WARNING]` would not re-import).
       expect(out.startsWith('> [!warning]\n')).toBe(true);
       expect(out).not.toContain('[!WARNING]');
-      // Both paragraph children are present, each blockquote-prefixed.
-      expect(out).toContain('> line1\n> line2');
+      // Both paragraph children are present, each blockquote-prefixed, blank-`>`
+      // separated so they stay distinct paragraphs on re-parse.
+      expect(out).toContain('> line1\n>\n> line2');
     });
 
     // Spec 4 — blockquote per-line prefixer over a multi-line nested callout.
@@ -607,13 +614,11 @@ describe('convertProseMirrorToMarkdown', () => {
           ],
         }),
       );
-      // NOTE(review): the spec predicted '> :::info\n> a\n>\n> b\n> :::',
-      // assuming the nested callout body contains a blank line between 'a' and
-      // The nested callout renders as an Obsidian callout '> [!info]\n> a\n> b'
-      // (single-'\n' join, no blank line). The outer blockquote prefixer then
-      // prefixes each of those lines with '> ' again, yielding a doubly-nested
-      // blockquote — the realistic per-line-prefix loop over a multi-line child.
-      expect(out).toBe('> > [!info]\n> > a\n> > b');
+      // The nested callout renders as an Obsidian callout '> [!info]\n> a\n>\n> b'
+      // (blank-`>` separated children per the #351 fix). The outer blockquote
+      // prefixer then prefixes each of those lines with '> ' again, yielding a
+      // doubly-nested blockquote — the per-line-prefix loop over a multi-line child.
+      expect(out).toBe('> > [!info]\n> > a\n> >\n> > b');
       // Every produced line carries the '> ' prefix (no line escapes to col 0).
       for (const line of out.split('\n')) {
         expect(line.startsWith('>')).toBe(true);