fix: two masters naming each other are a mirrored pair, not a title page

LibreOffice's Left Page / Right Page chain into each other through
style:next-style-name, which was read as the different-first-page idiom: one
master's geometry and zones governed the whole section, so a book-style body
lost its running head on every second page and its inner/outer margins with
it. A next-style cycle now becomes one mirrored setup — the right page's
geometry, the left page's zones as the even variant — and style:page-usage
left/right marks the margins as mirrored.

src/lib/import/odt.ts

@@ -904,7 +904,7 @@ function hfSetOfMasterPage(
   const numFormat = ctx.resolver.pageNumberFormat(hf.restPage ?? name);
   return {
     margins: rest,
-    marginsFirst: hf.restPage ? own : null,
+    marginsFirst: hf.restPage && !hf.mirrorPair ? own : null,
     pageNumberFormat: numFormat !== docNumFormat ? numFormat : null,
     format: paper && doc && paper.format !== doc.format ? paper.format : null,
     orientation: paper && doc && paper.orientation !== doc.orientation ? paper.orientation : null,

src/lib/import/styleResolver.ts

@@ -668,6 +668,7 @@ export class StyleResolver {
     footerBandCm: number;
     firstPageOnly: boolean;
     restPage: string | null;
+    mirrorPair: boolean;
   } {
     const mp = this.masterPageEl(pageName);
     const layout = this.pageLayoutEl(pageName);
@@ -688,11 +689,24 @@ export class StyleResolver {
     const successor = nextName && nextName !== mp?.getAttributeNS(NS.style, 'name')
       ? this.masterPageEl(nextName)
       : null;
-    const rest = successor ?? mp;
+    let rest = successor ?? mp;
+
+    // Two masters naming each other are the left/right pair of a mirrored book, not a
+    // hand-over: LibreOffice's own Left Page / Right Page. The right one governs the odd
+    // pages, the left one becomes the even variant, and neither ends the section.
+    const twin = this.mirrorTwin(rest);
+    // The master named here is itself one half of the pair — as opposed to a title page
+    // handing over *to* the pair, which really does govern one page only.
+    const pair = !!twin && (rest === mp || twin === mp);
+    const leftPage = twin
+      ? (this.pageUsage(rest) === 'left' ? rest : this.pageUsage(twin) === 'left' ? twin : null)
+      : null;
+    if (leftPage === rest) rest = twin;
+    const handover = pair ? null : successor;
 
     const zone = (local: string) => zoneIn(rest, local);
     const firstZone = (local: string): Element | null =>
-      zoneIn(mp, `${local}-first`) ?? (successor ? zoneIn(mp, local) : null);
+      zoneIn(mp, `${local}-first`) ?? (handover ? zoneIn(mp, local) : null);
     const bandProps = (local: 'header-style' | 'footer-style'): Element | null => {
       if (!layout) return null;
       for (const child of Array.from(layout.children)) {
@@ -729,21 +743,38 @@ export class StyleResolver {
       footer: zone('footer'),
       headerFirst: firstZone('header'),
       footerFirst: firstZone('footer'),
-      // Even-page variant (Word odd/even). ODF header-left = the left (even) page.
-      headerLeft: zoneIn(rest, 'header-left'),
-      footerLeft: zoneIn(rest, 'footer-left'),
+      // Even-page variant (Word odd/even). ODF header-left = the left (even) page; the
+      // left master of a mirrored pair is that variant in full.
+      headerLeft: zoneIn(rest, 'header-left') ?? zoneIn(leftPage, 'header'),
+      footerLeft: zoneIn(rest, 'footer-left') ?? zoneIn(leftPage, 'footer'),
       headerExtraCm: extraCm('header-style', 'margin-bottom'),
       footerExtraCm: extraCm('footer-style', 'margin-top'),
       headerBandCm: bandCm('header-style', 'margin-bottom'),
       footerBandCm: bandCm('footer-style', 'margin-top'),
       // Handing over to a successor is itself the "different first page" flag: a title
       // master with no zones of its own leaves page one deliberately blank.
-      firstPageOnly: !!successor,
+      firstPageOnly: !!handover,
       // The master every page after the first uses — also where their geometry comes from.
-      restPage: successor?.getAttributeNS(NS.style, 'name') ?? null,
+      restPage: rest !== mp ? rest?.getAttributeNS(NS.style, 'name') ?? null : null,
+      mirrorPair: pair,
     };
   }
 
+  private pageUsage(mp: Element | null): string {
+    const name = mp?.getAttributeNS(NS.style, 'name') ?? null;
+    return name ? this.pageLayoutEl(name)?.getAttributeNS(NS.style, 'page-usage') ?? '' : '';
+  }
+
+  // The master that names this one back through style:next-style-name — the other half
+  // of a left/right pair. A one-way chain is the title-page hand-over instead.
+  private mirrorTwin(mp: Element | null): Element | null {
+    const name = mp?.getAttributeNS(NS.style, 'name');
+    const next = mp?.getAttributeNS(NS.style, 'next-style-name');
+    if (!name || !next || next === name) return null;
+    const twin = this.masterPageEl(next);
+    return twin?.getAttributeNS(NS.style, 'next-style-name') === name ? twin : null;
+  }
+
   // Page margins + orientation + format from the master page's layout. With a
   // header/footer the body margin is page margin + zone height + spacing (inverse of
   // the export mapping). Format is matched from fo:page-width/height (fallback A4).
@@ -769,8 +800,13 @@ export class StyleResolver {
       left: cm('margin-left', 2.12),
       right: cm('margin-right', 2.12),
     };
-    if (this.pageLayoutEl(pageName)?.getAttributeNS(NS.style, 'page-usage') === 'mirrored') {
+    // A page style that is one side of a book (style:page-usage left/right) mirrors as
+    // surely as one marked "mirrored"; the editor stores the odd page's pair and swaps
+    // it on an even page, so a left-hand layout hands over its mirror image.
+    const usage = this.pageLayoutEl(pageName)?.getAttributeNS(NS.style, 'page-usage');
+    if (usage === 'mirrored' || usage === 'left' || usage === 'right') {
       margins.mirrored = true;
+      if (usage === 'left') [margins.left, margins.right] = [margins.right, margins.left];
     }
     const w = lengthToCm(props.getAttributeNS(NS.fo, 'page-width'));
     const h = lengthToCm(props.getAttributeNS(NS.fo, 'page-height'));

tests/unit/mirror-master-pair.test.ts

@@ -0,0 +1,61 @@
+import { describe, it, expect } from 'vitest';
+import { zipSync, strToU8 } from 'fflate';
+import { importOdt } from '../../src/lib/import/odt';
+
+// Two masters naming each other through style:next-style-name are LibreOffice's Left
+// Page / Right Page: one mirrored setup whose left half is the even-page variant, not a
+// title page handing over to a running one.
+const NS =
+  'xmlns:office="urn:oasis:names:tc:opendocument:xmlns:office:1.0" ' +
+  'xmlns:style="urn:oasis:names:tc:opendocument:xmlns:style:1.0" ' +
+  'xmlns:fo="urn:oasis:names:tc:opendocument:xmlns:xsl-fo-compatible:1.0" ' +
+  'xmlns:text="urn:oasis:names:tc:opendocument:xmlns:text:1.0"';
+
+function odt(chained: boolean): Uint8Array {
+  const layout = (name: string, usage: string, left: string, right: string) =>
+    `<style:page-layout style:name="${name}" style:page-usage="${usage}"><style:page-layout-properties
+      fo:page-width="21cm" fo:page-height="29.7cm" fo:margin-top="2cm" fo:margin-bottom="2cm"
+      fo:margin-left="${left}" fo:margin-right="${right}"/></style:page-layout>`;
+  const next = (n: string) => (chained ? ` style:next-style-name="${n}"` : '');
+  const styles = `<?xml version="1.0"?><office:document-styles ${NS}><office:styles>
+   <style:style style:name="Standard" style:family="paragraph"/>
+   <style:style style:name="Body" style:family="paragraph" style:master-page-name="Right"/>
+  </office:styles><office:automatic-styles>
+   ${layout('pmR', 'right', '3.6cm', '2.7cm')}${layout('pmL', 'left', '2.7cm', '3.6cm')}
+  </office:automatic-styles><office:master-styles>
+   <style:master-page style:name="Right" style:page-layout-name="pmR"${next('Left')}>
+    <style:header><text:p text:style-name="Standard">odd head</text:p></style:header></style:master-page>
+   <style:master-page style:name="Left" style:page-layout-name="pmL"${next('Right')}>
+    <style:header><text:p text:style-name="Standard">even head</text:p></style:header></style:master-page>
+  </office:master-styles></office:document-styles>`;
+  const content = `<?xml version="1.0"?><office:document-content ${NS}><office:body><office:text>
+   <text:p text:style-name="Standard">first</text:p>
+   <text:p text:style-name="Body">second</text:p>
+  </office:text></office:body></office:document-content>`;
+  return zipSync({ 'content.xml': strToU8(content), 'styles.xml': strToU8(styles) });
+}
+
+const zoneText = (doc: unknown): string =>
+  JSON.stringify(doc).match(/"text":"([^"]*)"/)?.[1] ?? '';
+
+describe('a left/right master pair', () => {
+  const section = importOdt(odt(true)).hfSections?.[1];
+
+  it('is one mirrored page setup, stored as the right (odd) page', () => {
+    expect(section?.margins).toEqual({ top: 2, bottom: 2, left: 3.6, right: 2.7, mirrored: true });
+    expect(section?.marginsFirst).toBeNull();
+  });
+
+  it('makes the left master the even-page variant', () => {
+    expect(section?.differentOddEven).toBe(true);
+    expect(section?.differentFirstPage).toBe(false);
+    expect(zoneText(section?.header)).toBe('odd head');
+    expect(zoneText(section?.headerEven)).toBe('even head');
+  });
+
+  it('leaves an unchained pair alone', () => {
+    const plain = importOdt(odt(false)).hfSections?.[1];
+    expect(plain?.differentOddEven).toBe(false);
+    expect(zoneText(plain?.header)).toBe('odd head');
+  });
+});