[layouts] Fix incorrect first page size in multipage layouts when

first page is skipped from output

Fixes #18742

src/core/layout/qgslayoutexporter.cpp

@@ -1024,7 +1024,7 @@ void QgsLayoutExporter::preparePrintAsPdf( QgsLayout *layout, QPrinter &printer,
   // Also an issue with PDF paper size using QPrinter::NativeFormat on Mac (always outputs portrait letter-size)
   printer.setOutputFormat( QPrinter::PdfFormat );
 
-  updatePrinterPageSize( layout, printer, 0 );
+  updatePrinterPageSize( layout, printer, firstPageToBeExported( layout ) );
 
   // TODO: add option for this in layout
   // May not work on Windows or non-X11 Linux. Works fine on Mac using QPrinter::NativeFormat
@@ -1043,7 +1043,7 @@ void QgsLayoutExporter::preparePrint( QgsLayout *layout, QPrinter &printer, bool
 
   if ( setFirstPageSize )
   {
-    updatePrinterPageSize( layout, printer, 0 );
+    updatePrinterPageSize( layout, printer, firstPageToBeExported( layout ) );
   }
 }
 
@@ -1566,6 +1566,21 @@ QImage QgsLayoutExporter::createImage( const QgsLayoutExporter::ImageExportSetti
   }
 }
 
+int QgsLayoutExporter::firstPageToBeExported( QgsLayout *layout )
+{
+  const int pageCount = layout->pageCollection()->pageCount();
+  for ( int i = 0; i < pageCount; ++i )
+  {
+    if ( !layout->pageCollection()->shouldExportPage( i ) )
+    {
+      continue;
+    }
+
+    return i;
+  }
+  return 0; // shouldn't really matter -- we aren't exporting ANY pages!
+}
+
 QString QgsLayoutExporter::generateFileName( const PageExportDetails &details ) const
 {
   if ( details.page == 0 )

src/core/layout/qgslayoutexporter.h

@@ -496,6 +496,12 @@ class CORE_EXPORT QgsLayoutExporter
 
     QImage createImage( const ImageExportSettings &settings, int page, QRectF &bounds, bool &skipPage ) const;
 
+    /**
+     * Returns the page number of the first page to be exported from the layout, skipping any pages
+     * which have been excluded from export.
+     */
+    static int firstPageToBeExported( QgsLayout *layout );
+
     /**
      * Saves an image to a file, possibly using format specific options (e.g. LZW compression for tiff)
     */

tests/src/python/test_qgslayoutexporter.py

@@ -463,6 +463,48 @@ class TestQgsLayoutExporter(unittest.TestCase):
         self.assertEqual(metadata['SUBJECT'], 'proj abstract')
         self.assertEqual(metadata['TITLE'], 'proj title')
 
+    def testExportToPdfSkipFirstPage(self):
+        l = QgsLayout(QgsProject.instance())
+        l.initializeDefaults()
+
+        # page 1 is excluded from export
+        page1 = l.pageCollection().page(0)
+        page1.setExcludeFromExports(True)
+
+        # add a second page
+        page2 = QgsLayoutItemPage(l)
+        page2.setPageSize('A5')
+        l.pageCollection().addPage(page2)
+
+        item2 = QgsLayoutItemShape(l)
+        item2.attemptSetSceneRect(QRectF(10, 20, 100, 150))
+        item2.attemptMove(QgsLayoutPoint(10, 20), page=1)
+        fill = QgsSimpleFillSymbolLayer()
+        fill_symbol = QgsFillSymbol()
+        fill_symbol.changeSymbolLayer(0, fill)
+        fill.setColor(Qt.cyan)
+        fill.setStrokeStyle(Qt.NoPen)
+        item2.setSymbol(fill_symbol)
+        l.addItem(item2)
+
+        exporter = QgsLayoutExporter(l)
+        # setup settings
+        settings = QgsLayoutExporter.PdfExportSettings()
+        settings.dpi = 80
+        settings.rasterizeWholeImage = False
+        settings.forceVectorOutput = False
+        settings.exportMetadata = True
+
+        pdf_file_path = os.path.join(self.basetestpath, 'test_exporttopdfdpi_skip_first.pdf')
+        self.assertEqual(exporter.exportToPdf(pdf_file_path, settings), QgsLayoutExporter.Success)
+        self.assertTrue(os.path.exists(pdf_file_path))
+
+        rendered_page_1 = os.path.join(self.basetestpath, 'test_exporttopdfdpi_skip_first.png')
+        dpi = 80
+        pdfToPng(pdf_file_path, rendered_page_1, dpi=dpi, page=1)
+
+        self.assertTrue(self.checkImage('test_exporttopdfdpi_skip_first', 'exporttopdfdpi_page2', rendered_page_1, size_tolerance=1))
+
     def testExportToSvg(self):
         md = QgsProject.instance().metadata()
         md.setTitle('proj title')