fix(rendering): apply superscript/subscript font-size scaling during layout

packages/layout-engine/contracts/src/index.ts

@@ -180,6 +180,10 @@ export type RunMarks = {
   highlight?: string;
   /** Text transformation (case modification). */
   textTransform?: 'uppercase' | 'lowercase' | 'capitalize' | 'none';
+  /** Vertical alignment for superscript/subscript text. */
+  vertAlign?: 'superscript' | 'subscript' | 'baseline';
+  /** Custom baseline shift in points (positive = raise, negative = lower). Takes precedence over vertAlign for positioning. */
+  baselineShift?: number;
 };
 
 export type TextRun = RunMarks & {

packages/layout-engine/painters/dom/src/renderer.ts

@@ -6543,6 +6543,8 @@ const deriveBlockVersion = (block: FlowBlock): string => {
           textRun.strike ? 1 : 0,
           textRun.highlight ?? '',
           textRun.letterSpacing != null ? textRun.letterSpacing : '',
+          textRun.vertAlign ?? '',
+          textRun.baselineShift != null ? textRun.baselineShift : '',
           // Note: pmStart/pmEnd intentionally excluded to prevent O(n) change detection
           textRun.token ?? '',
           // Tracked changes - force re-render when added or removed tracked change
@@ -6784,6 +6786,8 @@ const deriveBlockVersion = (block: FlowBlock): string => {
               hash = hashString(hash, getRunUnderlineStyle(run));
               hash = hashString(hash, getRunUnderlineColor(run));
               hash = hashString(hash, getRunBooleanProp(run, 'strike') ? '1' : '');
+              hash = hashString(hash, getRunStringProp(run, 'vertAlign'));
+              hash = hashNumber(hash, getRunNumberProp(run, 'baselineShift'));
             }
           }
         }
@@ -6882,6 +6886,17 @@ const applyRunStyles = (element: HTMLElement, run: Run, _isLink = false): void =
   if (decorations.length > 0) {
     element.style.textDecorationLine = decorations.join(' ');
   }
+
+  // Vertical alignment: custom baseline offset takes precedence over vertAlign
+  if (run.baselineShift != null && Number.isFinite(run.baselineShift)) {
+    element.style.verticalAlign = `${run.baselineShift}pt`;
+  } else if (run.vertAlign === 'superscript') {
+    element.style.verticalAlign = 'super';
+  } else if (run.vertAlign === 'subscript') {
+    element.style.verticalAlign = 'sub';
+  } else if (run.vertAlign === 'baseline') {
+    element.style.verticalAlign = 'baseline';
+  }
 };
 
 interface CommentHighlightResult {

packages/layout-engine/painters/dom/src/text-style-rendering.test.ts

@@ -322,6 +322,169 @@ describe('DomPainter text style CSS rendering', () => {
       expectCssColor(span?.style.color ?? '', '#ff0000');
     });
 
+    it('should apply vertical-align super for superscript', () => {
+      const block = createParagraphBlock('para-va-1', [
+        {
+          text: '1st',
+          fontFamily: 'Arial',
+          fontSize: 10.4,
+          vertAlign: 'superscript' as const,
+          pmStart: 0,
+          pmEnd: 3,
+        },
+      ]);
+
+      const measure = createParagraphMeasure();
+      const layout = createParagraphLayout('para-va-1');
+
+      const painter = createDomPainter({
+        blocks: [block],
+        measures: [measure],
+      });
+
+      painter.paint(layout, container);
+
+      const span = container.querySelector('span');
+      expect(span).toBeTruthy();
+      expect(span?.style.verticalAlign).toBe('super');
+    });
+
+    it('should apply vertical-align sub for subscript', () => {
+      const block = createParagraphBlock('para-va-2', [
+        {
+          text: '2',
+          fontFamily: 'Arial',
+          fontSize: 10.4,
+          vertAlign: 'subscript' as const,
+          pmStart: 0,
+          pmEnd: 1,
+        },
+      ]);
+
+      const measure = createParagraphMeasure();
+      const layout = createParagraphLayout('para-va-2');
+
+      const painter = createDomPainter({
+        blocks: [block],
+        measures: [measure],
+      });
+
+      painter.paint(layout, container);
+
+      const span = container.querySelector('span');
+      expect(span).toBeTruthy();
+      expect(span?.style.verticalAlign).toBe('sub');
+    });
+
+    it('should apply vertical-align with pt offset for baselineShift', () => {
+      const block = createParagraphBlock('para-va-3', [
+        {
+          text: 'shifted',
+          fontFamily: 'Arial',
+          fontSize: 16,
+          baselineShift: 3,
+          pmStart: 0,
+          pmEnd: 7,
+        },
+      ]);
+
+      const measure = createParagraphMeasure();
+      const layout = createParagraphLayout('para-va-3');
+
+      const painter = createDomPainter({
+        blocks: [block],
+        measures: [measure],
+      });
+
+      painter.paint(layout, container);
+
+      const span = container.querySelector('span');
+      expect(span).toBeTruthy();
+      expect(span?.style.verticalAlign).toBe('3pt');
+    });
+
+    it('should not apply vertical-align when neither vertAlign nor baselineShift is set', () => {
+      const block = createParagraphBlock('para-va-4', [
+        {
+          text: 'normal',
+          fontFamily: 'Arial',
+          fontSize: 16,
+          pmStart: 0,
+          pmEnd: 6,
+        },
+      ]);
+
+      const measure = createParagraphMeasure();
+      const layout = createParagraphLayout('para-va-4');
+
+      const painter = createDomPainter({
+        blocks: [block],
+        measures: [measure],
+      });
+
+      painter.paint(layout, container);
+
+      const span = container.querySelector('span');
+      expect(span).toBeTruthy();
+      expect(span?.style.verticalAlign).toBe('');
+    });
+
+    it('should use baselineShift over vertAlign when both are set', () => {
+      const block = createParagraphBlock('para-va-5', [
+        {
+          text: '1st',
+          fontFamily: 'Arial',
+          fontSize: 10.4,
+          vertAlign: 'superscript' as const,
+          baselineShift: 4,
+          pmStart: 0,
+          pmEnd: 3,
+        },
+      ]);
+
+      const measure = createParagraphMeasure();
+      const layout = createParagraphLayout('para-va-5');
+
+      const painter = createDomPainter({
+        blocks: [block],
+        measures: [measure],
+      });
+
+      painter.paint(layout, container);
+
+      const span = container.querySelector('span');
+      expect(span).toBeTruthy();
+      // baselineShift takes precedence — should be "4pt", not "super"
+      expect(span?.style.verticalAlign).toBe('4pt');
+    });
+
+    it('should apply negative baselineShift', () => {
+      const block = createParagraphBlock('para-va-6', [
+        {
+          text: 'lowered',
+          fontFamily: 'Arial',
+          fontSize: 16,
+          baselineShift: -2.5,
+          pmStart: 0,
+          pmEnd: 7,
+        },
+      ]);
+
+      const measure = createParagraphMeasure();
+      const layout = createParagraphLayout('para-va-6');
+
+      const painter = createDomPainter({
+        blocks: [block],
+        measures: [measure],
+      });
+
+      painter.paint(layout, container);
+
+      const span = container.querySelector('span');
+      expect(span).toBeTruthy();
+      expect(span?.style.verticalAlign).toBe('-2.5pt');
+    });
+
     it('should handle empty text with textTransform', () => {
       const block = createParagraphBlock('para-8', [
         {

packages/layout-engine/pm-adapter/src/attributes/paragraph.test.ts

@@ -153,4 +153,37 @@ describe('computeRunAttrs', () => {
 
     expect(result.vanish).toBe(true);
   });
+
+  it('passes through vertAlign', () => {
+    const result = computeRunAttrs({ vertAlign: 'superscript', fontSize: 24 } as never);
+    expect(result.vertAlign).toBe('superscript');
+  });
+
+  it('scales fontSize by 0.65 for superscript', () => {
+    const base = computeRunAttrs({ fontSize: 24 } as never);
+    const sup = computeRunAttrs({ fontSize: 24, vertAlign: 'superscript' } as never);
+    expect(sup.fontSize).toBeCloseTo(base.fontSize * 0.65);
+  });
+
+  it('scales fontSize by 0.65 for subscript', () => {
+    const base = computeRunAttrs({ fontSize: 24 } as never);
+    const sub = computeRunAttrs({ fontSize: 24, vertAlign: 'subscript' } as never);
+    expect(sub.fontSize).toBeCloseTo(base.fontSize * 0.65);
+  });
+
+  it('does not scale fontSize when position is set', () => {
+    const base = computeRunAttrs({ fontSize: 24 } as never);
+    const result = computeRunAttrs({ fontSize: 24, vertAlign: 'superscript', position: 6 } as never);
+    expect(result.fontSize).toBe(base.fontSize);
+  });
+
+  it('converts position from half-points to points as baselineShift', () => {
+    const result = computeRunAttrs({ position: 6 } as never);
+    expect(result.baselineShift).toBe(3);
+  });
+
+  it('does not set baselineShift when position is absent', () => {
+    const result = computeRunAttrs({ fontSize: 24 } as never);
+    expect(result.baselineShift).toBeUndefined();
+  });
 });

packages/layout-engine/pm-adapter/src/attributes/paragraph.ts

@@ -17,6 +17,7 @@ import type { PMNode } from '../types.js';
 import type { ResolvedRunProperties } from '@superdoc/word-layout';
 import { computeWordParagraphLayout } from '@superdoc/word-layout';
 import { pickNumber, twipsToPx, isFiniteNumber, ptToPx } from '../utilities.js';
+import { SUBSCRIPT_SUPERSCRIPT_SCALE } from '../constants.js';
 import { normalizeAlignment, normalizeParagraphSpacing } from './spacing-indent.js';
 import { normalizeOoxmlTabs } from './tabs.js';
 import { normalizeParagraphBorders, normalizeParagraphShading } from './borders.js';
@@ -319,9 +320,18 @@ export const computeRunAttrs = (
     fontFamily =
       runProps.fontFamily?.ascii || runProps.fontFamily?.hAnsi || runProps.fontFamily?.eastAsia || defaultFontFamily;
   }
+  const vertAlign = runProps.vertAlign as 'superscript' | 'subscript' | 'baseline' | undefined;
+  const hasPosition = runProps.position != null && Number.isFinite(runProps.position);
+  let fontSize = runProps.fontSize ? ptToPx(runProps.fontSize / 2)! : defaultFontSizePx;
+
+  // Scale font size for superscript/subscript when no custom position override
+  if (!hasPosition && (vertAlign === 'superscript' || vertAlign === 'subscript')) {
+    fontSize *= SUBSCRIPT_SUPERSCRIPT_SCALE;
+  }
+
   return {
     fontFamily: toCssFontFamily(fontFamily)!,
-    fontSize: runProps.fontSize ? ptToPx(runProps.fontSize / 2)! : defaultFontSizePx,
+    fontSize,
     bold: runProps.bold,
     italic: runProps.italic,
     underline:
@@ -339,5 +349,7 @@ export const computeRunAttrs = (
     letterSpacing: runProps.letterSpacing ? twipsToPx(runProps.letterSpacing) : undefined,
     lang: runProps.lang?.val || undefined,
     vanish: runProps.vanish,
+    vertAlign,
+    baselineShift: hasPosition ? runProps.position! / 2 : undefined,
   };
 };

packages/layout-engine/pm-adapter/src/constants.ts

@@ -5,6 +5,13 @@
 import type { TextRun, TrackedChangeKind } from '@superdoc/contracts';
 import type { HyperlinkConfig } from './types.js';
 
+/**
+ * Font size scaling factor for subscript and superscript text.
+ * Matches Microsoft Word's default rendering behavior for w:vertAlign
+ * when set to 'superscript' or 'subscript'.
+ */
+export const SUBSCRIPT_SUPERSCRIPT_SCALE = 0.65;
+
 /**
  * Unit conversion constants
  */

packages/layout-engine/pm-adapter/src/marks/application.test.ts

@@ -810,6 +810,76 @@ describe('mark application', () => {
         expect(run.textTransform).toBe('uppercase');
       });
     });
+
+    describe('vertAlign', () => {
+      it('sets vertAlign for superscript', () => {
+        const run: TextRun = { text: '1st', fontFamily: 'Arial', fontSize: 16 };
+        applyTextStyleMark(run, { vertAlign: 'superscript' });
+        expect(run.vertAlign).toBe('superscript');
+      });
+
+      it('sets vertAlign for subscript', () => {
+        const run: TextRun = { text: 'H2O', fontFamily: 'Arial', fontSize: 16 };
+        applyTextStyleMark(run, { vertAlign: 'subscript' });
+        expect(run.vertAlign).toBe('subscript');
+      });
+
+      it('sets vertAlign for baseline', () => {
+        const run: TextRun = { text: 'text', fontFamily: 'Arial', fontSize: 16 };
+        applyTextStyleMark(run, { vertAlign: 'baseline' });
+        expect(run.vertAlign).toBe('baseline');
+      });
+
+      it('ignores invalid vertAlign values', () => {
+        const run: TextRun = { text: 'text', fontFamily: 'Arial', fontSize: 16 };
+        applyTextStyleMark(run, { vertAlign: 'invalid' });
+        expect(run.vertAlign).toBeUndefined();
+      });
+
+      it('scales fontSize by 0.65 for superscript', () => {
+        const run: TextRun = { text: '1st', fontFamily: 'Arial', fontSize: 16 };
+        applyTextStyleMark(run, { vertAlign: 'superscript' });
+        expect(run.fontSize).toBeCloseTo(16 * 0.65);
+      });
+
+      it('scales fontSize by 0.65 for subscript', () => {
+        const run: TextRun = { text: 'H2O', fontFamily: 'Arial', fontSize: 16 };
+        applyTextStyleMark(run, { vertAlign: 'subscript' });
+        expect(run.fontSize).toBeCloseTo(16 * 0.65);
+      });
+
+      it('does not scale fontSize for baseline', () => {
+        const run: TextRun = { text: 'text', fontFamily: 'Arial', fontSize: 16 };
+        applyTextStyleMark(run, { vertAlign: 'baseline' });
+        expect(run.fontSize).toBe(16);
+      });
+
+      it('does not scale fontSize when baselineShift is set', () => {
+        const run: TextRun = { text: '1st', fontFamily: 'Arial', fontSize: 16 };
+        applyTextStyleMark(run, { vertAlign: 'superscript', position: '3pt' });
+        expect(run.fontSize).toBe(16);
+      });
+    });
+
+    describe('position / baselineShift', () => {
+      it('parses position string to baselineShift number', () => {
+        const run: TextRun = { text: 'text', fontFamily: 'Arial', fontSize: 16 };
+        applyTextStyleMark(run, { position: '3pt' });
+        expect(run.baselineShift).toBe(3);
+      });
+
+      it('handles negative position values', () => {
+        const run: TextRun = { text: 'text', fontFamily: 'Arial', fontSize: 16 };
+        applyTextStyleMark(run, { position: '-1.5pt' });
+        expect(run.baselineShift).toBe(-1.5);
+      });
+
+      it('ignores non-numeric position', () => {
+        const run: TextRun = { text: 'text', fontFamily: 'Arial', fontSize: 16 };
+        applyTextStyleMark(run, { position: 'invalid' });
+        expect(run.baselineShift).toBeUndefined();
+      });
+    });
   });
 
   describe('applyMarksToRun', () => {