IMSC 1.1: keep trailing <br/> forced line breaks

src/libse/SubtitleFormats/TimedTextImsc11.cs

@@ -626,7 +626,7 @@ public override void LoadSubtitle(Subtitle subtitle, List<string> lines, string
                     assStyle = GetAssStyleFromRegion(region.InnerText, headIndex);
                 }
 
-                var text = assStyle + ReadParagraph(node, headIndex).TrimEnd();
+                var text = assStyle + ReadParagraph(node, headIndex);
 
                 // Keep the region name: the alignment tag above only snaps to the screen thirds,
                 // the video preview positions the line from the region box itself.
@@ -637,9 +637,30 @@ public override void LoadSubtitle(Subtitle subtitle, List<string> lines, string
             subtitle.Renumber();
         }
 
+        /// <summary>
+        /// Reads one TTML <c>p</c> element into subtitle text. Trailing source whitespace is
+        /// trimmed here, but the line breaks a trailing <br/> adds are content: in TTML they are
+        /// forced breaks with layout meaning, so dropping them changes the cue (#15289). Nested
+        /// spans are read raw - only the finished paragraph is trimmed, so a space inside the
+        /// last span still survives.
+        /// </summary>
         private static string ReadParagraph(XmlNode node, TtmlHeadIndex headIndex)
+        {
+            var raw = BuildParagraph(node, headIndex, out var forcedBreakEnd);
+            if (forcedBreakEnd < 0)
+            {
+                return raw.TrimEnd();
+            }
+
+            // Everything before the last forced break already ends in a line break, so only the
+            // text behind it can be source formatting - that tail is trimmed as before.
+            return raw[..forcedBreakEnd] + raw[forcedBreakEnd..].TrimEnd();
+        }
+
+        private static string BuildParagraph(XmlNode node, TtmlHeadIndex headIndex, out int forcedBreakEnd)
         {
             var pText = new StringBuilder();
+            forcedBreakEnd = -1;
             foreach (XmlNode child in node.ChildNodes)
             {
                 if (child.NodeType == XmlNodeType.Text)
@@ -649,6 +670,7 @@ private static string ReadParagraph(XmlNode node, TtmlHeadIndex headIndex)
                 else if (child.Name == "br" || child.Name == "tt:br")
                 {
                     pText.AppendLine();
+                    forcedBreakEnd = pText.Length;
                 }
                 else if (child.Name == "#significant-whitespace" || child.Name == "tt:#significant-whitespace")
                 {
@@ -787,7 +809,13 @@ private static string ReadParagraph(XmlNode node, TtmlHeadIndex headIndex)
                         pText.Append(">");
                     }
 
-                    pText.Append(ReadParagraph(child, headIndex));
+                    var contentStart = pText.Length;
+                    var content = BuildParagraph(child, headIndex, out var nestedBreakEnd);
+                    pText.Append(content);
+                    if (nestedBreakEnd >= 0)
+                    {
+                        forcedBreakEnd = contentStart + nestedBreakEnd;
+                    }
 
                     if (!string.IsNullOrEmpty(fontFamily) || !string.IsNullOrEmpty(color))
                     {

tests/libse/SubtitleFormats/TimedTextImsc11Test.cs

@@ -128,4 +128,69 @@ public void IsMine_Accepts_All_Imsc_Extensions_And_Keeps_Consecutive_Line_Breaks
             Configuration.Settings.SubtitleSettings.TimedTextImsc11FileExtension = oldExtension;
         }
     }
+
+    private static string TrailingBreaksXml() =>
+        "<?xml version=\"1.0\" encoding=\"UTF-8\"?>" +
+        "<tt xmlns=\"http://www.w3.org/ns/ttml\" xmlns:ttp=\"http://www.w3.org/ns/ttml#parameter\" xml:lang=\"en\" ttp:profile=\"http://www.w3.org/ns/ttml/profile/imsc1/text\" ttp:timeBase=\"media\">" +
+        "<body><div>" +
+        "<p begin=\"00:00:01.000\" end=\"00:00:03.000\">A<br/><br/><br/>B</p>" +
+        "<p begin=\"00:00:04.000\" end=\"00:00:06.000\">C<br/><br/><br/></p>" +
+        "</div></body></tt>";
+
+    [Fact]
+    public void Trailing_Forced_Line_Breaks_Are_Preserved()
+    {
+        // #15289: the reader ran TrimEnd() over ReadParagraph(), so every trailing <br/> was
+        // dropped - three of them is a layout-significant blank line in TTML.
+        var sut = new TimedTextImsc11();
+        var subtitle = new Subtitle();
+
+        sut.LoadSubtitle(subtitle, TrailingBreaksXml().SplitToLines(), "test.xml");
+
+        Assert.Equal(2, subtitle.Paragraphs.Count);
+        Assert.Equal("A" + Environment.NewLine + Environment.NewLine + Environment.NewLine + "B",
+            subtitle.Paragraphs[0].Text);
+        Assert.Equal("C" + Environment.NewLine + Environment.NewLine + Environment.NewLine,
+            subtitle.Paragraphs[1].Text);
+    }
+
+    [Fact]
+    public void Trailing_Source_Whitespace_Is_Still_Trimmed()
+    {
+        // The TrimEnd() also kept source formatting out of the text: an inline trailing space and
+        // an indented paragraph must both come back without trailing whitespace.
+        var sut = new TimedTextImsc11();
+        var compact = TrailingBreaksXml().Replace("C<br/><br/><br/>", "C  ");
+        var indented = "<?xml version=\"1.0\" encoding=\"UTF-8\"?>" +
+                      "<tt xmlns=\"http://www.w3.org/ns/ttml\" xmlns:ttp=\"http://www.w3.org/ns/ttml#parameter\" xml:lang=\"en\" ttp:profile=\"http://www.w3.org/ns/ttml/profile/imsc1/text\" ttp:timeBase=\"media\">" +
+                      "<body><div>\n" +
+                      "  <p begin=\"00:00:01.000\" end=\"00:00:03.000\">Hello world</p>\n" +
+                      "</div></body></tt>";
+
+        var compactSubtitle = new Subtitle();
+        sut.LoadSubtitle(compactSubtitle, compact.SplitToLines(), "test.xml");
+        var indentedSubtitle = new Subtitle();
+        sut.LoadSubtitle(indentedSubtitle, indented.SplitToLines(), "test.xml");
+
+        Assert.Equal("C", compactSubtitle.Paragraphs[1].Text);
+        Assert.Equal("Hello world", indentedSubtitle.Paragraphs[0].Text);
+    }
+
+    [Fact]
+    public void LoadSaveLoad_Keeps_Trailing_Forced_Line_Breaks()
+    {
+        var sut = new TimedTextImsc11();
+        var subtitle = new Subtitle();
+        sut.LoadSubtitle(subtitle, TrailingBreaksXml().SplitToLines(), "test.xml");
+
+        var saved = sut.ToText(subtitle, "title");
+        var reloaded = new Subtitle();
+        new TimedTextImsc11().LoadSubtitle(reloaded, saved.SplitToLines(), "test.xml");
+
+        Assert.Equal(subtitle.Paragraphs.Count, reloaded.Paragraphs.Count);
+        Assert.Equal("A" + Environment.NewLine + Environment.NewLine + Environment.NewLine + "B",
+            reloaded.Paragraphs[0].Text);
+        Assert.Equal("C" + Environment.NewLine + Environment.NewLine + Environment.NewLine,
+            reloaded.Paragraphs[1].Text);
+    }
 }