Write rectangles with the precision of real numbers

src/foundation/src/PDFsharp/src/PdfSharp/Pdf.IO/PdfWriter.cs

@@ -236,7 +236,14 @@ public void Write(PdfLiteral value)
         /// </summary>
         public void Write(PdfRectangle rect)
         {
-            const string format = Config.SignificantDecimalPlaces3;
+            // With three decimal places, writing an existing page rounds its /MediaBox
+            // (e.g. 595.2756 -> 595.276). That is a change of the page geometry, which a validator
+            // reports as a changed page for every digital signature the document already contains.
+            //
+            // Note that the coordinates are written as they are, while Write(double) converts a real
+            // number to a single first. Doing that here would defeat the purpose: the single nearest to
+            // 595.2756 is 595.27557, which does not round-trip either.
+            const string format = Config.SignificantDecimalPlaces7;
             WriteSeparator(CharCat.Delimiter);
             WriteRaw(PdfEncoders.Format("[{0:" + format + "} {1:" + format + "} {2:" + format + "} {3:" + format + "}]", rect.X1, rect.Y1, rect.X2, rect.Y2));
         }

src/foundation/src/PDFsharp/tests/PdfSharp.Tests/IO/WriterTests.cs

@@ -29,5 +29,32 @@ public void Write_import_file()
             Action save = () => doc.Save(filename);
             save.Should().Throw<InvalidOperationException>();
         }
+
+        [Fact]
+        public void Write_rectangle_with_the_precision_of_a_real_number()
+        {
+            // ISO A4 in points. The values have four decimal places, which must survive a round-trip,
+            // because a changed /MediaBox is a changed page for every digital signature of the document.
+            const double width = 595.2756, height = 841.8898;
+
+            using var stream = new MemoryStream();
+            using (var document = new PdfDocument())
+            {
+                var page = document.AddPage();
+                page.MediaBox = new PdfRectangle(new XPoint(0, 0), new XPoint(width, height));
+                document.Save(stream, false);
+            }
+
+            var pdf = stream.ToArray();
+            var chars = new char[pdf.Length];
+            for (int idx = 0; idx < pdf.Length; idx++)
+                chars[idx] = (char)pdf[idx];
+            new String(chars).Should().Contain("[0 0 595.2756 841.8898]");
+
+            using var writtenDocument = PdfReader.Open(new MemoryStream(pdf), PdfDocumentOpenMode.Import);
+            var mediaBox = writtenDocument.Pages[0].MediaBox;
+            mediaBox.X2.Should().Be(width);
+            mediaBox.Y2.Should().Be(height);
+        }
     }
 }