fix: emit mirrorMargins in sectPr for Word for Mac PDF export (#74)

csharp-version/src/MarkdownToDocx.Core/OpenXml/OpenXmlDocumentBuilder.cs

@@ -109,6 +109,16 @@
             Gutter = (uint)pageConfig.GutterMargin
         });
 
+        // Word for Mac's "Save as PDF" / "Print to PDF" pipeline does not honor
+        // <w:mirrorMargins/> when it is only placed in settings.xml (spec-compliant
+        // location). Duplicating it inside sectPr makes the export pipeline mirror
+        // verso pages correctly while remaining harmless for Word/Windows and
+        // LibreOffice, which ignore unknown sectPr children. See issue #74.
+        if (pageConfig.MirrorMargins)
+        {
+            sectionProps.AppendChild(new W.MirrorMargins());
+        }
+
         return sectionProps;
     }
 
@@ -862,7 +872,7 @@
         // Adding indent equal to BorderSpace * 20 twips anchors the border at the margin boundary.
         if (style.BorderSpace > 0)
         {
             string indentTwips = (style.BorderSpace * 20).ToString();
             props.AppendChild(new Indentation { Left = indentTwips, Right = indentTwips });
         }
 
@@ -1266,7 +1276,7 @@
         var indentation = new Indentation { Left = style.LeftIndent };
         if (hasPadding && style.PaddingSpace > 0)
         {
             indentation.Right = (style.PaddingSpace * 20).ToString();
         }
         props.AppendChild(indentation);
 

csharp-version/tests/MarkdownToDocx.Tests/Integration/MarkdownToDocxIntegrationTests.cs

@@ -362,6 +362,103 @@ public void ConvertMarkdownTable_WithMirrorMargins_TableShouldUsePercentageWidth
             "table width must be percentage-based to prevent margin overflow on narrow pages");
     }
 
+    [Fact]
+    public void Build_WithMirrorMargins_ShouldEmitMirrorMarginsInBothSettingsAndSectPr()
+    {
+        // Regression test for issue #74: Word for Mac's "Save as PDF" pipeline
+        // does not honor <w:mirrorMargins/> when it is only placed in settings.xml.
+        // The element must also be duplicated inside sectPr so verso pages are
+        // mirrored correctly during PDF export.
+
+        // Arrange: PageLayout with MirrorMargins enabled (KDP paperback style).
+        var pageLayout = new PageLayoutConfig
+        {
+            Width = 14.8,
+            Height = 21.0,
+            MarginTop = 1.9,
+            MarginBottom = 1.9,
+            MarginLeft = 1.30,
+            MarginRight = 1.30,
+            MarginGutter = 1.20,
+            MirrorMargins = true
+        };
+
+        // Act
+        using var stream = new MemoryStream();
+        using (var builder = new OpenXmlDocumentBuilder(
+            stream,
+            new MarkdownToDocx.Styling.TextDirection.ConfigurableTextDirectionProvider(
+                new HorizontalTextProvider(), pageLayout)))
+        {
+            builder.Save();
+        }
+
+        // Assert: mirrorMargins present in both locations.
+        stream.Position = 0;
+        using var doc = WordprocessingDocument.Open(stream, false);
+
+        var settings = doc.MainDocumentPart!.DocumentSettingsPart!.Settings;
+        settings.Elements<MirrorMargins>().Should().HaveCount(1,
+            "spec-compliant location (CT_Settings) must keep emitting mirrorMargins");
+
+        var sectionProps = doc.MainDocumentPart.Document.Body!
+            .Elements<SectionProperties>().LastOrDefault();
+        sectionProps.Should().NotBeNull();
+        // mirrorMargins is not part of CT_SectPr in OOXML schema, so on
+        // deserialization OpenXml SDK exposes it as an OpenXmlUnknownElement
+        // (rather than the typed MirrorMargins class). Match by local name to
+        // confirm the raw XML carries it inside sectPr.
+        bool sectPrCarriesMirrorMargins = sectionProps!.ChildElements
+            .Any(e => e.LocalName == "mirrorMargins"
+                   && e.NamespaceUri == "http://schemas.openxmlformats.org/wordprocessingml/2006/main");
+        sectPrCarriesMirrorMargins.Should().BeTrue(
+            "Word for Mac PDF export requires mirrorMargins inside sectPr (issue #74)");
+    }
+
+    [Fact]
+    public void Build_WithoutMirrorMargins_ShouldNotEmitMirrorMarginsAnywhere()
+    {
+        // Arrange: PageLayout with MirrorMargins disabled.
+        var pageLayout = new PageLayoutConfig
+        {
+            Width = 14.8,
+            Height = 21.0,
+            MarginTop = 1.9,
+            MarginBottom = 1.9,
+            MarginLeft = 1.30,
+            MarginRight = 1.30,
+            MirrorMargins = false
+        };
+
+        // Act
+        using var stream = new MemoryStream();
+        using (var builder = new OpenXmlDocumentBuilder(
+            stream,
+            new MarkdownToDocx.Styling.TextDirection.ConfigurableTextDirectionProvider(
+                new HorizontalTextProvider(), pageLayout)))
+        {
+            builder.Save();
+        }
+
+        // Assert: no mirrorMargins emitted in either location.
+        stream.Position = 0;
+        using var doc = WordprocessingDocument.Open(stream, false);
+
+        doc.MainDocumentPart!.DocumentSettingsPart.Should().BeNull(
+            "settings.xml part must not be created when MirrorMargins is false");
+
+        var sectionProps = doc.MainDocumentPart.Document.Body!
+            .Elements<SectionProperties>().LastOrDefault();
+        sectionProps.Should().NotBeNull();
+        // Match by local name (OpenXml SDK deserializes unknown sectPr children
+        // as OpenXmlUnknownElement; see Build_WithMirrorMargins for context).
+        bool sectPrCarriesMirrorMargins = sectionProps!.ChildElements
+            .Any(e => e.LocalName == "mirrorMargins"
+                   && e.NamespaceUri == "http://schemas.openxmlformats.org/wordprocessingml/2006/main");
+        sectPrCarriesMirrorMargins.Should().BeFalse(
+            "sectPr must not carry mirrorMargins when feature is disabled");
+    }
+
     public void Dispose()
     {
         try