Fix external subtitle stream mapping for multi-stream containers

Compute the in-file stream index for external subtitles instead of
hardcoding -map 1:0. For single-stream files (SRT/ASS/VTT) the index
is always 0, preserving existing behavior. For multi-stream containers
like MKS, the correct track is selected by counting sibling streams
that share the same Path.

Add unit tests for GetMapArgs covering internal subs, external SRT,
multiple external files, and multi-stream MKS containers.

1 files changed, 13 insertions, 2 deletions

diff --git a/MediaBrowser.Controller/MediaEncoding/EncodingHelper.cs b/MediaBrowser.Controller/MediaEncoding/EncodingHelper.cs
index 1fdb5fd4bd..40bb3913e9 100644
--- a/MediaBrowser.Controller/MediaEncoding/EncodingHelper.cs
+++ b/MediaBrowser.Controller/MediaEncoding/EncodingHelper.cs
@@ -3104,8 +3104,19 @@ namespace MediaBrowser.Controller.MediaEncoding
             {
                 if (state.SubtitleStream.IsExternal)
                 {
-                    // External subtitle file is added as second FFmpeg input
-                    args += " -map 1:0";
+                    // External subtitle file is added as second FFmpeg input.
+                    // For single-stream files (SRT/ASS/VTT) the in-file index is always 0.
+                    // For multi-stream containers (MKS) we count how many streams from
+                    // the same file appear before the selected one.
+                    var inFileIndex = state.MediaSource.MediaStreams
+                        .Where(s => string.Equals(s.Path, state.SubtitleStream.Path, StringComparison.Ordinal))
+                        .TakeWhile(s => s.Index != state.SubtitleStream.Index)
+                        .Count();
+
+                    args += string.Format(
+                        CultureInfo.InvariantCulture,
+                        " -map 1:{0}",
+                        inFileIndex);
                 }
                 else
                 {