Merge pull request 'fix(converter): стабильность round-trip image/медиа — «» ≡ absent (класс defaults-instability)' (#350) from fix/media-roundtrip-stability into develop

Reviewed-on: #350

packages/prosemirror-markdown/src/lib/docmost-schema.ts

@@ -635,13 +635,17 @@ const Attachment = Node.create({
       },
       name: {
         default: null,
-        parseHTML: (el: HTMLElement) => el.getAttribute("data-attachment-name"),
+        // Empty-string-vs-absent idempotency (GS-EDIT-REVERT class): "" -> default.
+        parseHTML: (el: HTMLElement) =>
+          el.getAttribute("data-attachment-name") || null,
         renderHTML: (attrs: Record<string, any>) =>
           attrs.name ? { "data-attachment-name": attrs.name } : {},
       },
       mime: {
         default: null,
-        parseHTML: (el: HTMLElement) => el.getAttribute("data-attachment-mime"),
+        // Empty-string-vs-absent idempotency (GS-EDIT-REVERT class): "" -> default.
+        parseHTML: (el: HTMLElement) =>
+          el.getAttribute("data-attachment-mime") || null,
         renderHTML: (attrs: Record<string, any>) =>
           attrs.mime ? { "data-attachment-mime": attrs.mime } : {},
       },
@@ -689,7 +693,10 @@ const Video = Node.create({
       },
       alt: {
         default: null,
-        parseHTML: (el: HTMLElement) => el.getAttribute("aria-label"),
+        // Empty-string-vs-absent idempotency: coerce "" back to the default so a
+        // stray empty `aria-label` never materializes `alt: ""` on a video stored
+        // with no alt (same GS-EDIT-REVERT class as the image `alt` fix).
+        parseHTML: (el: HTMLElement) => el.getAttribute("aria-label") || null,
         renderHTML: (attrs: Record<string, any>) =>
           attrs.alt ? { "aria-label": attrs.alt } : {},
       },
@@ -864,13 +871,15 @@ const diagramAttributes = () => ({
   },
   title: {
     default: null,
-    parseHTML: (el: HTMLElement) => el.getAttribute("data-title"),
+    // Empty-string-vs-absent idempotency (GS-EDIT-REVERT class): "" -> default.
+    parseHTML: (el: HTMLElement) => el.getAttribute("data-title") || null,
     renderHTML: (attrs: Record<string, any>) =>
       attrs.title ? { "data-title": attrs.title } : {},
   },
   alt: {
     default: null,
-    parseHTML: (el: HTMLElement) => el.getAttribute("data-alt"),
+    // Empty-string-vs-absent idempotency (GS-EDIT-REVERT class): "" -> default.
+    parseHTML: (el: HTMLElement) => el.getAttribute("data-alt") || null,
     renderHTML: (attrs: Record<string, any>) =>
       attrs.alt ? { "data-alt": attrs.alt } : {},
   },
@@ -1106,7 +1115,8 @@ const Pdf = Node.create({
       },
       name: {
         default: null,
-        parseHTML: (el: HTMLElement) => el.getAttribute("data-name"),
+        // Empty-string-vs-absent idempotency (GS-EDIT-REVERT class): "" -> default.
+        parseHTML: (el: HTMLElement) => el.getAttribute("data-name") || null,
         renderHTML: (attrs: Record<string, any>) =>
           attrs.name ? { "data-name": attrs.name } : {},
       },
@@ -1491,6 +1501,29 @@ export const docmostExtensions = [
           ...parent.height,
           parseHTML: (el: HTMLElement) => el.getAttribute("height"),
         },
+        // Empty-string-vs-absent idempotency (GS-EDIT-REVERT class). `marked`
+        // renders `![](src)` as `<img alt="">`, so the stock Image `alt`
+        // parseHTML (`getAttribute("alt")`) materializes `alt: ""` on an image
+        // that was stored with NO alt (attr absent). That is a false diff against
+        // the editor-stored form (a no-alt image has alt ABSENT, not ""), so a
+        // git-sync / ai-chat touch of a page with a plain image produced phantom
+        // churn. Coerce an empty string back to the attr's default (null) so the
+        // import is idempotent. A real alt survives verbatim (`|| undefined` keeps
+        // the truthy value; the default fills the empty case). `title` is coerced
+        // the same way for the whole class, even though `marked` does not
+        // currently emit `title=""` — defence in depth against any path that does.
+        // NOTE: this DIVERGES from editor-ext's literal image `alt` parseHTML
+        // (`getAttribute("alt")`, which returns "" verbatim), but CONVERGES on
+        // editor-ext's real STORED shape: an editor image inserted without alt
+        // renders with no `alt` attribute and re-parses as absent, never "".
+        alt: {
+          ...parent.alt,
+          parseHTML: (el: HTMLElement) => el.getAttribute("alt") || null,
+        },
+        title: {
+          ...parent.title,
+          parseHTML: (el: HTMLElement) => el.getAttribute("title") || null,
+        },
       };
     },
   }).configure({ inline: false }),

packages/prosemirror-markdown/test/roundtrip-stability.helper.ts

@@ -0,0 +1,443 @@
+/**
+ * Reusable round-trip-STABILITY matrix helper (fixtures-first).
+ *
+ * A single stored node authored WITHOUT a given string attribute (attr
+ * absent / undefined) must not gain a phantom EMPTY-STRING value after a
+ * markdown round-trip — the "empty-string-vs-absent" churn class. This helper,
+ * given a node spec, drives a matrix of attribute combinations through the REAL
+ * converter (`convertProseMirrorToMarkdown` -> `markdownToProseMirror`) and
+ * asserts byte-stability on two contours:
+ *
+ *   1. RAW round-trip: for the node under test, every attribute the round-trip
+ *      materializes must equal what the INPUT authored — an authored attr keeps
+ *      its value, an ABSENT attr may only reappear at its SCHEMA DEFAULT. If an
+ *      absent attr comes back as a NON-default value (e.g. `alt: ""` where the
+ *      default is `null`), that is an instability and is reported precisely as
+ *      `type.attr: absent -> "<got>"`. This is the contour git-sync / stored
+ *      JSON diffs on, so masking it only in `canonicalize` would leave the noise.
+ *
+ *   2. CANONICAL round-trip: `canonicalizeContent(original)` must deep-equal
+ *      `canonicalizeContent(roundtrip)` (a second, semantic contour).
+ *
+ * The ONLY normalization the helper treats as allowed (not an instability) is
+ * the DOCUMENTED numeric width/height/size/aspectRatio -> string coercion the
+ * converter performs on purpose (a stored numeric `640` re-parses via
+ * `getAttribute` as the string `"640"`). It is encoded here as an explicit
+ * per-spec `numericStringAttrs` set applied to BOTH contours, NOT a silent skip.
+ *
+ * The helper is node-type agnostic: image and the whole media family share the
+ * `align !== "center"` predicate + `<!--name {…}-->` comment machinery, so one
+ * matrix guards the shared class.
+ */
+import { getSchema } from "@tiptap/core";
+import {
+  convertProseMirrorToMarkdown,
+  markdownToProseMirror,
+  canonicalizeContent,
+  docmostExtensions,
+} from "../src/lib/index.js";
+import { firstDivergence } from "./roundtrip-helpers.js";
+
+/** One attribute's two probe values. */
+export interface AttrMatrixEntry {
+  /** Attribute name on the node. */
+  attr: string;
+  /**
+   * The "default" pick. `undefined` means the attribute is OMITTED entirely
+   * (the absent case — the one that can materialize an empty string on import).
+   * A concrete value is authored verbatim.
+   */
+  default: unknown;
+  /** A representative NON-default value to exercise (must survive verbatim). */
+  nonDefault: unknown;
+  /**
+   * Marks the attr as a member of the EMPTY-STRING class the fix targets: a
+   * string attr whose schema default is `null`/absent and whose parseHTML
+   * coerces `"" -> default` (image/drawio `alt`+`title`, video `alt` via
+   * aria-label, pdf/attachment `name`, attachment `mime`). Set true to also
+   * drive the THIRD-STATE convergence case (see runConvergenceCase) for this
+   * attr. Attrs whose default is NOT null (e.g. embed `provider`, default "")
+   * or that are not `""`-coerced (control attrs) are left unset.
+   */
+  emptyStringClass?: boolean;
+}
+
+/** A node type + the attribute matrix to sweep for it. */
+export interface NodeStabilitySpec {
+  /** Node type (e.g. "image", "video"). */
+  type: string;
+  /** Attributes always present on the node (e.g. `{ src: "/i.png" }`). */
+  baseAttrs?: Record<string, unknown>;
+  /** Attributes to sweep at default and non-default. */
+  attrMatrix: AttrMatrixEntry[];
+  /**
+   * Attributes whose numeric -> string coercion on round-trip is DOCUMENTED and
+   * intentional; compared modulo `String(x)` on both sides. Defaults to the
+   * converter's known sizing set.
+   */
+  numericStringAttrs?: string[];
+}
+
+/** A single unstable finding, legible enough to tie a gate-lock to. */
+export interface Instability {
+  type: string;
+  attr: string;
+  /** What the input authored: the literal value, or the ABSENT sentinel. */
+  authored: unknown | typeof ABSENT;
+  /** What the round-trip produced. */
+  got: unknown;
+  /** What a stable round-trip should have produced (authored value or default). */
+  expected: unknown;
+}
+
+/** One matrix cell's result. */
+export interface ComboResult {
+  label: string;
+  authored: Record<string, unknown>;
+  /** RAW-contour instabilities on the node under test. */
+  raw: Instability[];
+  /** CANONICAL-contour divergence (path + values) or null when equal. */
+  canonical: { path: string; a: unknown; b: unknown } | null;
+  /** True when the node type failed to round-trip at all (structural loss). */
+  missing: boolean;
+  md: string;
+}
+
+/** Whole-matrix report for one node spec. */
+export interface MatrixReport {
+  type: string;
+  combos: ComboResult[];
+}
+
+/** Sentinel marking an attribute the input did NOT author. */
+export const ABSENT = Symbol("ABSENT");
+
+const DEFAULT_NUMERIC_STRING_ATTRS = [
+  "width",
+  "height",
+  "size",
+  "aspectRatio",
+];
+
+// The ProseMirror schema the converter targets — its attribute `default`s are
+// the authoritative "what an absent attr should re-materialize as" oracle.
+const schema = getSchema(docmostExtensions);
+
+/** Read the schema default for every attribute of a node type. */
+function schemaDefaults(type: string): Record<string, unknown> {
+  const specAttrs = (schema.nodes[type]?.spec?.attrs ?? {}) as Record<
+    string,
+    { default: unknown }
+  >;
+  const out: Record<string, unknown> = {};
+  for (const [k, v] of Object.entries(specAttrs)) out[k] = v.default;
+  return out;
+}
+
+/** Find the first node of a given type anywhere in a PM doc tree. */
+function findFirst(node: any, type: string): any {
+  if (node && node.type === type) return node;
+  for (const child of node?.content ?? []) {
+    const hit = findFirst(child, type);
+    if (hit) return hit;
+  }
+  return null;
+}
+
+/** Coerce a scalar for the documented numeric->string comparison. */
+const numStr = (x: unknown): unknown => (x == null ? x : String(x));
+
+/**
+ * Enumerate the cartesian product of the matrix: every attribute independently
+ * at its default (index 0) or non-default (index 1) pick. The all-default
+ * corner is included (the baseline). Small by construction (2^N over a handful
+ * of at-risk string attrs).
+ */
+function enumerateCombos(matrix: AttrMatrixEntry[]): number[][] {
+  let combos: number[][] = [[]];
+  for (let i = 0; i < matrix.length; i++) {
+    const next: number[][] = [];
+    for (const c of combos) {
+      next.push([...c, 0]);
+      next.push([...c, 1]);
+    }
+    combos = next;
+  }
+  return combos;
+}
+
+/** Build the authored attrs for one combo pick vector. */
+function authoredAttrs(
+  spec: NodeStabilitySpec,
+  picks: number[],
+): Record<string, unknown> {
+  const attrs: Record<string, unknown> = { ...(spec.baseAttrs ?? {}) };
+  spec.attrMatrix.forEach((entry, i) => {
+    if (picks[i] === 1) {
+      attrs[entry.attr] = entry.nonDefault;
+    } else if (entry.default !== undefined) {
+      attrs[entry.attr] = entry.default;
+    }
+    // default === undefined -> OMIT the attr entirely (the absent case).
+  });
+  return attrs;
+}
+
+/** Human-readable label for a combo (which attrs are at non-default). */
+function comboLabel(spec: NodeStabilitySpec, picks: number[]): string {
+  const on = spec.attrMatrix
+    .filter((_, i) => picks[i] === 1)
+    .map((e) => e.attr);
+  return on.length === 0 ? "<all-default>" : on.join("+");
+}
+
+/**
+ * Run the full stability matrix for one node spec and return a structured
+ * report (does NOT throw — the caller asserts, so a failure can print the whole
+ * report). Every combo runs the real export->import pipeline once.
+ */
+export async function runStabilityMatrix(
+  spec: NodeStabilitySpec,
+): Promise<MatrixReport> {
+  const numericStringAttrs = new Set(
+    spec.numericStringAttrs ?? DEFAULT_NUMERIC_STRING_ATTRS,
+  );
+  const defaults = schemaDefaults(spec.type);
+  const combos: ComboResult[] = [];
+
+  for (const picks of enumerateCombos(spec.attrMatrix)) {
+    const authored = authoredAttrs(spec, picks);
+    const doc = { type: "doc", content: [{ type: spec.type, attrs: authored }] };
+    const md = convertProseMirrorToMarkdown(doc);
+    const rt = await markdownToProseMirror(md);
+    const node = findFirst(rt, spec.type);
+
+    const result: ComboResult = {
+      label: comboLabel(spec, picks),
+      authored,
+      raw: [],
+      canonical: null,
+      missing: node == null,
+      md,
+    };
+
+    if (node != null) {
+      // RAW contour: every materialized attr must equal the authored value, or
+      // (for an absent attr) the schema default — modulo the documented numeric
+      // string coercion.
+      const rtAttrs = (node.attrs ?? {}) as Record<string, unknown>;
+      for (const key of Object.keys(rtAttrs)) {
+        const authoredHas = Object.prototype.hasOwnProperty.call(authored, key);
+        const expected = authoredHas ? authored[key] : defaults[key];
+        let got = rtAttrs[key];
+        let exp = expected;
+        if (numericStringAttrs.has(key)) {
+          got = numStr(got);
+          exp = numStr(exp);
+        }
+        if (firstDivergence(got, exp) !== null) {
+          result.raw.push({
+            type: spec.type,
+            attr: key,
+            authored: authoredHas ? authored[key] : ABSENT,
+            got: rtAttrs[key],
+            expected,
+          });
+        }
+      }
+
+      // CANONICAL contour: canonical forms deep-equal, modulo the same numeric
+      // string coercion (applied to both trees so a documented coercion is not
+      // counted as a divergence).
+      const ca = normalizeNumeric(canonicalizeContent(doc), numericStringAttrs);
+      const cb = normalizeNumeric(canonicalizeContent(rt), numericStringAttrs);
+      result.canonical = firstDivergence(ca, cb);
+    }
+
+    combos.push(result);
+  }
+
+  return { type: spec.type, combos };
+}
+
+/**
+ * Deep-copy a canonical tree, coercing the documented numeric->string attrs to
+ * their string form so an intentional `640 -> "640"` coercion is not reported
+ * as a canonical divergence. Only touches the listed attribute keys.
+ */
+function normalizeNumeric(node: any, attrs: Set<string>): any {
+  if (Array.isArray(node)) return node.map((n) => normalizeNumeric(n, attrs));
+  if (node === null || typeof node !== "object") return node;
+  const out: Record<string, unknown> = {};
+  for (const key of Object.keys(node)) {
+    if (key === "attrs" && node.attrs && typeof node.attrs === "object") {
+      const a: Record<string, unknown> = {};
+      for (const [k, v] of Object.entries(node.attrs)) {
+        a[k] = attrs.has(k) ? numStr(v) : v;
+      }
+      out.attrs = a;
+    } else {
+      out[key] = normalizeNumeric(node[key], attrs);
+    }
+  }
+  return out;
+}
+
+/** Flatten a report to just its unstable combos (for a terse assertion). */
+export function unstableCombos(report: MatrixReport): ComboResult[] {
+  return report.combos.filter(
+    (c) => c.missing || c.raw.length > 0 || c.canonical !== null,
+  );
+}
+
+// ---------------------------------------------------------------------------
+// THIRD STATE: an EXPLICITLY-STORED empty string on a string attr.
+//
+// The matrix above sweeps TWO states per string attr: absent/default and a
+// non-default value — and asserts FIRST-pass byte-stability for both. There is
+// a third, degenerate state the matrix does NOT cover: the attr stored as a
+// LITERAL `""`. This is DISTINCT from "the node never had the attr": a user
+// types an alt in the editor, then deletes it, and Tiptap's
+// `updateAttributes({ alt: "" })` persists a literal `alt: ""` in the stored
+// JSON. There is no absent-vs-"" distinction in the DOM once serialized, so the
+// fix's `getAttribute("alt") || null` coercion canonicalizes BOTH to the
+// default (`null`).
+//
+// Consequence — and this is CORRECT, not a bug: a doc carrying an explicit `""`
+// converges to the default on the FIRST round-trip (a ONE-TIME diff: `"" ->
+// null`), then is byte-stable from the SECOND round-trip on (idempotent). So
+// this state must be pinned with a DIFFERENT contract than the matrix's:
+//   - do NOT assert first-pass byte-stability (the first pass legitimately
+//     changes `""` -> default), and
+//   - DO assert the first pass converges to the default AND the second pass is
+//     idempotent (rt2 deep-equals rt1).
+//
+// A future sync/QA pass diffing stored pages will see this one-time `"" -> null`
+// normalization exactly once per affected node; it is the converter canon, not
+// corruption, and must not be flagged as data loss.
+// ---------------------------------------------------------------------------
+
+/** Result of the third-state ("explicit empty string") convergence probe. */
+export interface ConvergenceResult {
+  type: string;
+  attr: string;
+  /** The schema default the attr must converge to on pass 1 (null / absent). */
+  expectedDefault: unknown;
+  /** rt1's materialized value for the attr — must equal `expectedDefault`. */
+  firstPassValue: unknown;
+  /** True when the node round-tripped AND rt1 converged the attr to default. */
+  convergedToDefault: boolean;
+  /** rt1-vs-rt2 divergence; MUST be null (idempotent from pass 2 on). */
+  secondPassDivergence: { path: string; a: unknown; b: unknown } | null;
+  /** True when the node type failed to round-trip at all (structural loss). */
+  missing: boolean;
+}
+
+/** Round-trip a full PM doc through the real converter once. */
+async function roundtripDoc(doc: any): Promise<any> {
+  return markdownToProseMirror(convertProseMirrorToMarkdown(doc));
+}
+
+/**
+ * Third-state convergence probe for one string attr of the empty-string class.
+ *
+ * (a) builds a doc with the attr EXPLICITLY set to `""` (baseAttrs + `""`),
+ * (b) rt1 = roundtrip(doc); asserts rt1's attr equals the schema default — the
+ *     documented ONE-TIME `"" -> default` normalization (NOT byte-stable vs the
+ *     `""` input, so first-pass stability is deliberately NOT asserted here),
+ * (c) rt2 = roundtrip(rt1); asserts rt2 deep-equals rt1 — idempotent from the
+ *     second round-trip on.
+ *
+ * Returns a structured result (does NOT throw) so the caller can assert and
+ * print. Reusable across the whole node family: drive it for every attr flagged
+ * `emptyStringClass` on every spec (see convergenceCasesFor / the test driver).
+ */
+export async function runConvergenceCase(
+  spec: NodeStabilitySpec,
+  attr: string,
+): Promise<ConvergenceResult> {
+  const expectedDefault = schemaDefaults(spec.type)[attr];
+
+  // (a) The degenerate third state: attr persisted as a LITERAL "".
+  const authored = { ...(spec.baseAttrs ?? {}), [attr]: "" };
+  const doc = { type: "doc", content: [{ type: spec.type, attrs: authored }] };
+
+  // (b) First round-trip: "" must normalize to the default (a one-time diff).
+  const rt1 = await roundtripDoc(doc);
+  const node1 = findFirst(rt1, spec.type);
+  const firstPassValue = node1?.attrs?.[attr];
+  const convergedToDefault =
+    node1 != null && firstDivergence(firstPassValue, expectedDefault) === null;
+
+  // (c) Second round-trip: must be byte-stable (rt2 deep-equals rt1). We compare
+  // the WHOLE docs — both are converter OUTPUTS already in the same materialized
+  // form (numeric attrs are strings on both sides), so no numeric normalization
+  // is needed here, unlike the raw/canonical contours above.
+  const rt2 = node1 != null ? await roundtripDoc(rt1) : rt1;
+  const secondPassDivergence =
+    node1 != null ? firstDivergence(rt1, rt2) : null;
+
+  return {
+    type: spec.type,
+    attr,
+    expectedDefault,
+    firstPassValue,
+    convergedToDefault,
+    secondPassDivergence,
+    missing: node1 == null,
+  };
+}
+
+/** The attrs of a spec flagged as members of the empty-string class. */
+export function convergenceCasesFor(spec: NodeStabilitySpec): string[] {
+  return spec.attrMatrix
+    .filter((e) => e.emptyStringClass)
+    .map((e) => e.attr);
+}
+
+/** True when a convergence result honours the "converges once, then stable" contract. */
+export function convergenceOk(r: ConvergenceResult): boolean {
+  return !r.missing && r.convergedToDefault && r.secondPassDivergence === null;
+}
+
+/** Render a convergence result as a legible one-liner for a failed assertion. */
+export function formatConvergence(r: ConvergenceResult): string {
+  if (r.missing) return `${r.type}.${r.attr}: DID-NOT-ROUND-TRIP`;
+  const parts: string[] = [];
+  if (!r.convergedToDefault) {
+    parts.push(
+      `pass1 did NOT converge: got ${JSON.stringify(r.firstPassValue)} (expected default ${JSON.stringify(r.expectedDefault)})`,
+    );
+  }
+  if (r.secondPassDivergence) {
+    parts.push(
+      `pass2 NOT idempotent @ ${r.secondPassDivergence.path}: ${JSON.stringify(r.secondPassDivergence.a)} vs ${JSON.stringify(r.secondPassDivergence.b)}`,
+    );
+  }
+  const status = parts.length === 0 ? "converges-once-then-stable" : parts.join("; ");
+  return `${r.type}.${r.attr}: ${status}`;
+}
+
+/** Render a report as a legible multi-line string for a failed assertion. */
+export function formatReport(report: MatrixReport): string {
+  const lines: string[] = [`node "${report.type}":`];
+  for (const c of report.combos) {
+    const flags: string[] = [];
+    if (c.missing) flags.push("DID-NOT-ROUND-TRIP");
+    for (const i of c.raw) {
+      const authored =
+        i.authored === ABSENT ? "absent" : JSON.stringify(i.authored);
+      flags.push(
+        `RAW ${i.type}.${i.attr}: ${authored} -> ${JSON.stringify(i.got)} (expected ${JSON.stringify(i.expected)})`,
+      );
+    }
+    if (c.canonical) {
+      flags.push(
+        `CANON @ ${c.canonical.path}: ${JSON.stringify(c.canonical.a)} vs ${JSON.stringify(c.canonical.b)}`,
+      );
+    }
+    const status = flags.length === 0 ? "stable" : flags.join("; ");
+    lines.push(`  [${c.label}] ${status}`);
+  }
+  return lines.join("\n");
+}

packages/prosemirror-markdown/test/roundtrip-stability.test.ts

@@ -0,0 +1,164 @@
+import { describe, expect, it } from "vitest";
+import {
+  runStabilityMatrix,
+  unstableCombos,
+  formatReport,
+  runConvergenceCase,
+  convergenceCasesFor,
+  convergenceOk,
+  formatConvergence,
+  type NodeStabilitySpec,
+} from "./roundtrip-stability.helper.js";
+
+// ---------------------------------------------------------------------------
+// Round-trip STABILITY matrix for image + the media family.
+//
+// Guards the "empty-string-vs-absent" churn class (GS-EDIT-REVERT family): a
+// stored node authored WITHOUT a string attr (alt/title/caption/aria-label/...)
+// must not gain a phantom `attr: ""` after `markdownToProseMirror(convert…)`.
+// Each spec sweeps the at-risk string attrs at DEFAULT (absent) and at a real
+// NON-default value; the helper asserts both the RAW round-trip (attrs equal the
+// input's, modulo the documented numeric width/height/size/aspectRatio -> string
+// coercion) and the CANONICAL round-trip (canonical forms deep-equal).
+//
+// The image + media family share the `align !== "center"` predicate and the
+// `<!--name {…}-->` comment machinery, so one matrix guards the shared class.
+// align is NOT part of this class (it round-trips correctly) and is not swept.
+// ---------------------------------------------------------------------------
+
+const SPECS: NodeStabilitySpec[] = [
+  {
+    // Image carries the most at-risk string attrs. `alt` is the one marked
+    // materializes as `<img alt="">` on `![](src)` import (the real bug); title
+    // and caption are covered as the same class. attachmentId is a string attr
+    // that must stay absent when unset (control).
+    type: "image",
+    baseAttrs: { src: "/i.png" },
+    attrMatrix: [
+      { attr: "alt", default: undefined, nonDefault: "a real alt text", emptyStringClass: true },
+      { attr: "title", default: undefined, nonDefault: "a real title", emptyStringClass: true },
+      { attr: "caption", default: undefined, nonDefault: "a real caption" },
+      { attr: "attachmentId", default: undefined, nonDefault: "att-42" },
+    ],
+  },
+  {
+    // Video's `alt` rides the `aria-label` attribute (media aria-label at risk).
+    type: "video",
+    baseAttrs: { src: "/v.mp4" },
+    attrMatrix: [
+      { attr: "alt", default: undefined, nonDefault: "a clip", emptyStringClass: true },
+      { attr: "attachmentId", default: undefined, nonDefault: "att-1" },
+    ],
+  },
+  {
+    // Audio carries no alt/title; attachmentId is its only optional string attr.
+    type: "audio",
+    baseAttrs: { src: "/a.mp3" },
+    attrMatrix: [
+      { attr: "attachmentId", default: undefined, nonDefault: "att-2" },
+    ],
+  },
+  {
+    // pdf: link-form media. `name` (filename) is its at-risk string attr.
+    type: "pdf",
+    baseAttrs: { src: "/d.pdf" },
+    attrMatrix: [
+      { attr: "name", default: undefined, nonDefault: "report.pdf", emptyStringClass: true },
+      { attr: "attachmentId", default: undefined, nonDefault: "att-3" },
+    ],
+  },
+  {
+    // attachment: link-form media (file card). `name` + `mime` string attrs.
+    type: "attachment",
+    baseAttrs: { url: "/f.zip" },
+    attrMatrix: [
+      { attr: "name", default: undefined, nonDefault: "bundle.zip", emptyStringClass: true },
+      { attr: "mime", default: undefined, nonDefault: "application/zip", emptyStringClass: true },
+      { attr: "attachmentId", default: undefined, nonDefault: "att-4" },
+    ],
+  },
+  {
+    // embed: link-form media. `provider` is its at-risk string attr (schema
+    // default ""). embed's numeric width/height defaults (800/600) are a SEPARATE,
+    // documented limitation OUTSIDE the empty-string class: they are not in
+    // canonicalize's KNOWN_DEFAULTS, so an ABSENT width/height re-imports as the
+    // 800/600 default and diverges canonically (see the note in canonicalize.ts).
+    // That is canonicalize-owned and out of scope here, so we author the
+    // dimensions at their defaults (as real editor embeds carry them) to keep this
+    // guard focused on the empty-string/provider class.
+    // provider's schema default is "" (NOT null), so a re-imported "" is the
+    // correct value, not a phantom — it is outside the null-default empty-string
+    // class. We author it at its "" default (the default pick) so the sweep still
+    // asserts a non-default provider ("youtube") round-trips, without tripping the
+    // canonicalize KNOWN_DEFAULTS gap for embed's non-null defaults.
+    type: "embed",
+    baseAttrs: { src: "https://example.com/x", width: 800, height: 600 },
+    attrMatrix: [
+      { attr: "provider", default: "", nonDefault: "youtube" },
+    ],
+  },
+  {
+    // drawio: image-form diagram. `title` + `alt` string attrs (data-title/-alt).
+    type: "drawio",
+    baseAttrs: { src: "blob:drawio" },
+    attrMatrix: [
+      { attr: "title", default: undefined, nonDefault: "flow chart", emptyStringClass: true },
+      { attr: "alt", default: undefined, nonDefault: "an alt", emptyStringClass: true },
+      { attr: "attachmentId", default: undefined, nonDefault: "att-5" },
+    ],
+  },
+  {
+    // excalidraw: image-form diagram, same shared diagramAttributes set.
+    type: "excalidraw",
+    baseAttrs: { src: "blob:excalidraw" },
+    attrMatrix: [
+      { attr: "title", default: undefined, nonDefault: "sketch", emptyStringClass: true },
+      { attr: "alt", default: undefined, nonDefault: "an alt", emptyStringClass: true },
+      { attr: "attachmentId", default: undefined, nonDefault: "att-6" },
+    ],
+  },
+];
+
+describe("round-trip stability matrix (image + media family)", () => {
+  for (const spec of SPECS) {
+    it(`${spec.type}: no attr materializes an empty-string / phantom value`, async () => {
+      const report = await runStabilityMatrix(spec);
+      const unstable = unstableCombos(report);
+      // On failure, print the WHOLE matrix so which (attr, value) combos are
+      // unstable is legible.
+      expect(unstable, `\n${formatReport(report)}\n`).toEqual([]);
+    });
+  }
+});
+
+// ---------------------------------------------------------------------------
+// THIRD STATE: an attr EXPLICITLY stored as a literal "" (GS-EDIT-REVERT: a user
+// typed alt/title/name/... then deleted it, so Tiptap persisted `attr: ""` — a
+// value DISTINCT from "attr was never set"). Unlike the absent case above, this
+// state is NOT first-pass byte-stable: the fix's `"" -> default` coercion is a
+// deliberate ONE-TIME normalization on the FIRST sync round-trip, stable
+// thereafter. We therefore assert a DIFFERENT contract — "converges to default
+// on pass 1, then idempotent from pass 2 on" — for every empty-string-class attr
+// across the whole node family (image/video/pdf/attachment/drawio/excalidraw).
+//
+// IMPORTANT for a future sync/QA pass: the pass-1 `"" -> null` diff is the
+// converter canon, not corruption. It appears at most once per affected node and
+// must NOT be flagged as "the converter is losing/corrupting page data".
+// ---------------------------------------------------------------------------
+describe("round-trip third state: explicit empty string converges once, then idempotent", () => {
+  for (const spec of SPECS) {
+    for (const attr of convergenceCasesFor(spec)) {
+      it(`${spec.type}.${attr}: "" normalizes to default on pass 1, byte-stable from pass 2`, async () => {
+        const r = await runConvergenceCase(spec, attr);
+        // Pass 1 must converge "" -> the schema default (the one-time diff) and
+        // pass 2 (roundtrip of pass-1 output) must be byte-stable. formatConvergence
+        // prints exactly which half failed.
+        expect(convergenceOk(r), `\n${formatConvergence(r)}\n`).toBe(true);
+        // Spell the contract out explicitly so the intent is legible in the test:
+        expect(r.convergedToDefault, `\n${formatConvergence(r)}\n`).toBe(true);
+        expect(r.firstPassValue).toEqual(r.expectedDefault);
+        expect(r.secondPassDivergence, `\n${formatConvergence(r)}\n`).toBeNull();
+      });
+    }
+  }
+});