Only pass muxer private options to the muxer they belong to

`-movflags` (mov/mp4 family) and `-default_mode` (matroska family) were
added to every ffmpeg command regardless of the output format, so e.g.
exporting to matroska produced:

    ... -movflags "+faststart" -default_mode infer_no_subs -f matroska ...

ffmpeg tolerates private options belonging to a different muxer, so this
was harmless, but it adds noise to the command line we log, show under
"Last commands" and include in problem reports, which makes
troubleshooting harder than it needs to be.

Gate each set of flags on the output format. The format lists are derived
from ffmpeg's movenc.c and matroskaenc.c rather than the existing
util/streams.ts `isMov`/`isMatroska` helpers, because those are narrower
UI oriented lists that omit movenc formats we can output (3gp, 3g2, f4v,
psp) - reusing them would have silently dropped +faststart for those.

When the output format is unknown, ffmpeg infers the muxer from the file
extension, so the flags are kept as before.

Co-Authored-By: Claude <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PqUn6Bh7hWqPqWejqR8p7u

src/renderer/src/hooks/useFfmpegOperations.ts

@@ -41,7 +41,19 @@ async function writeChaptersFfmetadata(outDir: string, chapters: Chapter[] | und
   return path;
 }
 
-function getMovFlags({ preserveMovData, movFastStart }: { preserveMovData: boolean, movFastStart: boolean }) {
+// Muxers implemented by ffmpeg's movenc.c, i.e. the ones that accept `-movflags`.
+// Note: deliberately not util/streams.ts `isMov`, which is a narrower, UI oriented list.
+const movencFormats = new Set(['3g2', '3gp', 'f4v', 'ipod', 'ismv', 'mov', 'mp4', 'psp']);
+
+// Muxers implemented by ffmpeg's matroskaenc.c, i.e. the ones that accept `-default_mode`.
+const matroskaencFormats = new Set(['matroska', 'webm']);
+
+// ffmpeg tolerates private options belonging to a different muxer, but they add noise to the command line
+// that we log, show in "Last commands" and include in error reports - which makes troubleshooting harder.
+// If the output format is unknown, ffmpeg infers the muxer from the file extension, so keep the flags to be safe.
+function getMovFlags({ outFormat, preserveMovData, movFastStart }: { outFormat: string | undefined, preserveMovData: boolean, movFastStart: boolean }) {
+  if (outFormat != null && !movencFormats.has(outFormat)) return [];
+
   const flags: string[] = [];
 
   // https://video.stackexchange.com/a/26084/29486
@@ -55,7 +67,10 @@ function getMovFlags({ preserveMovData, movFastStart }: { preserveMovData: boole
   return flags.flatMap((flag) => ['-movflags', flag]);
 }
 
-function getMatroskaFlags() {
+// same as getMovFlags, but for the matroska muxer's private options
+function getMatroskaFlags(outFormat: string | undefined) {
+  if (outFormat != null && !matroskaencFormats.has(outFormat)) return [];
+
   return [
     '-default_mode', 'infer_no_subs',
     // because it makes sense to not force subtitles disposition to "default" if they were not default in the input file
@@ -201,8 +216,8 @@ function useFfmpegOperations({ filePath, treatInputFileModifiedTimeAsStart, trea
 
         ...(chaptersInputIndex != null ? ['-map_chapters', String(chaptersInputIndex)] : []),
 
-        ...getMovFlags({ preserveMovData, movFastStart }),
-        ...getMatroskaFlags(),
+        ...getMovFlags({ outFormat, preserveMovData, movFastStart }),
+        ...getMatroskaFlags(outFormat),
 
         // See https://github.com/mifi/lossless-cut/issues/170
         '-ignore_unknown',
@@ -454,8 +469,8 @@ function useFfmpegOperations({ filePath, treatInputFileModifiedTimeAsStart, trea
 
       ...(shortestFlag ? ['-shortest'] : []),
 
-      ...getMovFlags({ preserveMovData, movFastStart }),
-      ...getMatroskaFlags(),
+      ...getMovFlags({ outFormat, preserveMovData, movFastStart }),
+      ...getMatroskaFlags(outFormat),
 
       ...customFileMetadataArgs,
 
@@ -1081,6 +1096,7 @@ function useFfmpegOperations({ filePath, treatInputFileModifiedTimeAsStart, trea
     customOutDir: string | undefined, streams: FFprobeStream[],
   }) => {
     invariant(filePath != null);
+    await assertFileExists(filePath);
 
     const attachmentStreams = streams.filter((s) => s.codec_type === 'attachment');
     const nonAttachmentStreams = streams.filter((s) => s.codec_type !== 'attachment');