fix(diffing): gate paragraph identity on structural depth (IT-1065)

Diff replay was overwriting table-cell content with body-paragraph text
when a comparison contained both a structural change (table removed)
and trailing text changes. DOCX importers renumber w14:paraId values
across structural changes, so cell-paragraph IDs in the source can
collide with top-level paragraph IDs in the target. The flat sequence
diff then paired them as the same paragraph and emitted modification
ops anchored inside the table.

Gate paragraphComparator and canTreatAsModification on depth equality
before any identity signal. Cross-depth content-signature matches were
the same hazard for blank or repeated-label paragraphs.

Add a fixture pair derived from the IT-1065 reporter's documents and
four unit tests covering paraId, content-signature, and similarity
cross-depth rejection.

Author: caio-pizzol

packages/super-editor/src/editors/v1/extensions/diffing/algorithm/paragraph-diffing.test.js

@@ -159,6 +159,35 @@ describe('paragraphComparator', () => {
   it('returns false for paragraphs with different identity signals', () => {
     expect(paragraphComparator({ fullText: 'one' }, { fullText: 'two' })).toBe(false);
   });
+
+  it('rejects paraId match across different depths', () => {
+    // Regression: importers (e.g., Google Docs) renumber paraIds when structure
+    // changes, so a table-cell paragraph (deeper) can collide with a body
+    // paragraph (depth 0). Treat them as different paragraphs.
+    const oldCell = createParagraphInfo({
+      depth: 3,
+      fullText: 'Milestone',
+      node: createParagraphNode({ attrs: { paraId: '00000008' } }),
+    });
+    const newBody = createParagraphInfo({
+      depth: 0,
+      fullText: '',
+      node: createParagraphNode({ attrs: { paraId: '00000008' } }),
+    });
+    expect(paragraphComparator(oldCell, newBody)).toBe(false);
+  });
+
+  it('rejects content-signature match across different depths', () => {
+    const oldCell = createParagraphInfo({ depth: 3, fullText: '' });
+    const newBody = createParagraphInfo({ depth: 0, fullText: '' });
+    expect(paragraphComparator(oldCell, newBody)).toBe(false);
+  });
+
+  it('still matches by content signature at the same depth', () => {
+    const oldParagraph = createParagraphInfo({ depth: 0, fullText: 'shared content' });
+    const newParagraph = createParagraphInfo({ depth: 0, fullText: 'shared content' });
+    expect(paragraphComparator(oldParagraph, newParagraph)).toBe(true);
+  });
 });
 
 describe('paragraph diff builders', () => {
@@ -488,4 +517,13 @@ describe('canTreatAsModification', () => {
     const b = { node: { attrs: {} }, fullText: 'dolor sit' };
     expect(canTreatAsModification(a, b)).toBe(false);
   });
+
+  it('rejects similarity-based pairing across different depths', () => {
+    // Even with text similar enough to clear SIMILARITY_THRESHOLD, paragraphs
+    // at different structural depths should not be re-paired as a single
+    // modification - their replay anchor would land in the wrong context.
+    const a = createParagraphInfo({ depth: 3, fullText: 'lorem ipsum dolor' });
+    const b = createParagraphInfo({ depth: 0, fullText: 'lorem ipsum dolor!' });
+    expect(canTreatAsModification(a, b)).toBe(false);
+  });
 });

packages/super-editor/src/editors/v1/extensions/diffing/algorithm/paragraph-diffing.ts

@@ -145,8 +145,16 @@ export function shouldProcessEqualAsModification(
  *
  * The content signature covers both text and inline nodes (images, etc.),
  * so image-only paragraphs with different images are not falsely paired.
+ *
+ * Identity is gated on structural context (depth) first. Cross-depth pairing
+ * via paraId or content-signature alone produces false identity when DOCX
+ * importers renumber paraIds across structural changes, e.g. a table-cell
+ * paragraph and a body paragraph sharing a paraId or both being empty.
  */
 export function paragraphComparator(oldParagraph: ParagraphNodeInfo, newParagraph: ParagraphNodeInfo): boolean {
+  if (oldParagraph?.depth !== newParagraph?.depth) {
+    return false;
+  }
   const oldId = oldParagraph?.node?.attrs?.paraId;
   const newId = newParagraph?.node?.attrs?.paraId;
   if (oldId && newId && oldId === newId) {
@@ -223,6 +231,9 @@ export function buildModifiedParagraphDiff(
  * Decides whether a delete/insert pair should be reinterpreted as a modification to minimize noisy diff output.
  */
 export function canTreatAsModification(oldParagraph: ParagraphNodeInfo, newParagraph: ParagraphNodeInfo): boolean {
+  if (oldParagraph?.depth !== newParagraph?.depth) {
+    return false;
+  }
   if (paragraphComparator(oldParagraph, newParagraph)) {
     return true;
   }

packages/super-editor/src/editors/v1/extensions/diffing/replayDiffs.test.js

@@ -382,6 +382,11 @@ const getReplayFixturePairs = () => [
   ['diff_before7.docx', 'diff_after7.docx'],
   ['diff_before8.docx', 'diff_after8.docx'],
   ['diff_before9.docx', 'diff_after9.docx'],
+  // IT-1065: combined structural (table removal) + text change. Google Docs
+  // renumbers paraIds across the structural change, so cell paragraphs in
+  // before share paraIds with body paragraphs in after. Depth-gated identity
+  // prevents false matches.
+  ['diff_before_it1065.docx', 'diff_after_it1065.docx'],
 ];
 
 /**