Normalize VobSub .sub to .idx for embedding, add EncodingHelper tests

Move the .sub to .idx path normalization outside the burn-in check so
it applies to subtitle embedding as well. ffmpeg requires the .idx file
to read VobSub subtitles.

Add unit tests for GetMapArgs and GetInputArgument covering internal
subs, external SRT, multi-file SRT, multi-stream MKS containers, and
VobSub .sub/.idx path normalization.

2 files changed, 51 insertions, 12 deletions

diff --git a/MediaBrowser.Controller/MediaEncoding/EncodingHelper.cs b/MediaBrowser.Controller/MediaEncoding/EncodingHelper.cs
index 40bb3913e9..6ff2873cc5 100644
--- a/MediaBrowser.Controller/MediaEncoding/EncodingHelper.cs
+++ b/MediaBrowser.Controller/MediaEncoding/EncodingHelper.cs
@@ -1272,18 +1272,14 @@ namespace MediaBrowser.Controller.MediaEncoding
                 var subtitlePath = state.SubtitleStream.Path;
                 var isGraphicalBurnIn = ShouldEncodeSubtitle(state) && !state.SubtitleStream.IsTextSubtitleStream;
 
-                if (isGraphicalBurnIn)
+                // dvdsub/vobsub graphical subtitles use .sub+.idx pairs
+                var subtitleExtension = Path.GetExtension(subtitlePath.AsSpan());
+                if (subtitleExtension.Equals(".sub", StringComparison.OrdinalIgnoreCase))
                 {
-                    var subtitleExtension = Path.GetExtension(subtitlePath.AsSpan());
-
-                    // dvdsub/vobsub graphical subtitles use .sub+.idx pairs
-                    if (subtitleExtension.Equals(".sub", StringComparison.OrdinalIgnoreCase))
+                    var idxFile = Path.ChangeExtension(subtitlePath, ".idx");
+                    if (File.Exists(idxFile))
                     {
-                        var idxFile = Path.ChangeExtension(subtitlePath, ".idx");
-                        if (File.Exists(idxFile))
-                        {
-                            subtitlePath = idxFile;
-                        }
+                        subtitlePath = idxFile;
                     }
                 }
 
diff --git a/tests/Jellyfin.Controller.Tests/MediaEncoding/EncodingHelperTests.cs b/tests/Jellyfin.Controller.Tests/MediaEncoding/EncodingHelperTests.cs
index e6276f0e9e..6563dd63ab 100644
--- a/tests/Jellyfin.Controller.Tests/MediaEncoding/EncodingHelperTests.cs
+++ b/tests/Jellyfin.Controller.Tests/MediaEncoding/EncodingHelperTests.cs
@@ -1,11 +1,13 @@
 using System;
 using System.Collections.Generic;
+using System.IO;
 using Jellyfin.Data.Enums;
 using MediaBrowser.Common.Configuration;
 using MediaBrowser.Controller.IO;
 using MediaBrowser.Controller.Library;
 using MediaBrowser.Controller.MediaEncoding;
 using MediaBrowser.Controller.Streaming;
+using MediaBrowser.Model.Configuration;
 using MediaBrowser.Model.Dlna;
 using MediaBrowser.Model.Dto;
 using MediaBrowser.Model.Entities;
@@ -71,6 +73,8 @@ namespace Jellyfin.Controller.Tests.MediaEncoding
         [Fact]
         public void GetMapArgs_SecondExternalSrt_StillMaps1Colon0()
         {
+            // Two separate .srt files — selecting the second one still maps 1:0
+            // because Jellyfin feeds only the selected file as ffmpeg input 1.
             var ext1 = new MediaStream
             {
                 Index = 2,
@@ -92,7 +96,6 @@ namespace Jellyfin.Controller.Tests.MediaEncoding
             var state = BuildState(ext2, SubtitleDeliveryMethod.Embed, additionalStreams: [ext1, ext2]);
             var args = CreateHelper().GetMapArgs(state);
 
-            // Different file from ext1, so in-file index is 0
             Assert.Contains("-map 1:0", args, StringComparison.Ordinal);
         }
 
@@ -156,10 +159,50 @@ namespace Jellyfin.Controller.Tests.MediaEncoding
             var state = BuildState(mks1, SubtitleDeliveryMethod.Embed, additionalStreams: [mks0, mks1, mks2]);
             var args = CreateHelper().GetMapArgs(state);
 
-            // Second track in the same .mks file → in-file index 1
             Assert.Contains("-map 1:1", args, StringComparison.Ordinal);
         }
 
+        [Theory]
+        [InlineData(SubtitleDeliveryMethod.Embed, true, "movie.idx")]
+        [InlineData(SubtitleDeliveryMethod.Encode, true, "movie.idx")]
+        [InlineData(SubtitleDeliveryMethod.Embed, false, "movie.sub")]
+        [InlineData(SubtitleDeliveryMethod.Encode, false, "movie.sub")]
+        public void GetInputArgument_VobSub_UsesCorrectPath(
+            SubtitleDeliveryMethod deliveryMethod,
+            bool createIdxFile,
+            string expectedFilename)
+        {
+            var tempDir = Directory.CreateTempSubdirectory("jellyfin-test-");
+            try
+            {
+                var subFile = Path.Combine(tempDir.FullName, "movie.sub");
+                File.WriteAllText(subFile, "dummy");
+
+                if (createIdxFile)
+                {
+                    File.WriteAllText(Path.Combine(tempDir.FullName, "movie.idx"), "dummy");
+                }
+
+                var sub = new MediaStream
+                {
+                    Index = 2,
+                    Type = MediaStreamType.Subtitle,
+                    Codec = "dvdsub",
+                    IsExternal = true,
+                    SupportsExternalStream = true,
+                    Path = subFile
+                };
+                var state = BuildState(sub, deliveryMethod);
+                var inputArgs = CreateHelper().GetInputArgument(state, new EncodingOptions(), null);
+
+                Assert.Contains(expectedFilename, inputArgs, StringComparison.Ordinal);
+            }
+            finally
+            {
+                tempDir.Delete(true);
+            }
+        }
+
         private static EncodingJobInfo BuildState(
             MediaStream? subtitle,
             SubtitleDeliveryMethod? deliveryMethod,