re-encode flac when cutting

and warn about it
fixes #1809

src/renderer/src/components/ExportConfirm.tsx

@@ -103,6 +103,14 @@ function ExportConfirm({
   // some thumbnail streams (png,jpg etc) cannot always be cut correctly, so we warn if they try to.
   const areWeCuttingProblematicStreams = areWeCutting && mainCopiedThumbnailStreams.length > 0;
 
+  const warnings = useMemo(() => {
+    const ret: string[] = [];
+    // https://github.com/mifi/lossless-cut/issues/1809
+    if (areWeCutting && outFormat === 'flac') {
+      ret.push(t('There is a known issue in FFmpeg with cutting FLAC files. The file will be re-encoded, which is still lossless, but the export may be slower.'));
+    }
+    return ret;
+  }, [areWeCutting, outFormat, t]);
   const exportModeDescription = useMemo(() => ({
     segments_to_chapters: t('Don\'t cut the file, but instead export an unmodified original which has chapters generated from segments'),
     merge: t('Auto merge segments to one file after export'),
@@ -208,6 +216,14 @@ function ExportConfirm({
 
                 <table className={styles['options']}>
                   <tbody>
+                    {warnings.map((warning) => (
+                      <tr key={warning}>
+                        <td colSpan={2}>
+                          <div style={warningStyle}><WarningSignIcon verticalAlign="middle" color="warning" /> {warnings.join('\n')}</div>
+                        </td>
+                        <td />
+                      </tr>
+                    ))}
                     {selectedSegments.length !== nonFilteredSegmentsOrInverse.length && (
                       <tr>
                         <td colSpan={2}>

src/renderer/src/hooks/useFfmpegOperations.ts

@@ -218,7 +218,8 @@ function useFfmpegOperations({ filePath, treatInputFileModifiedTimeAsStart, trea
     const cuttingStart = isCuttingStart(cutFrom);
     const cutFromWithAdjustment = cutFrom + cutFromAdjustmentFrames * frameDuration;
     const cuttingEnd = isCuttingEnd(cutTo, videoDuration);
-    console.log('Cutting from', cuttingStart ? `${cutFrom} (${cutFromWithAdjustment} adjusted ${cutFromAdjustmentFrames} frames)` : 'start', 'to', cuttingEnd ? cutTo : 'end');
+    const areWeCutting = cuttingStart || cuttingEnd;
+    if (areWeCutting) console.log('Cutting from', cuttingStart ? `${cutFrom} (${cutFromWithAdjustment} adjusted ${cutFromAdjustmentFrames} frames)` : 'start', 'to', cuttingEnd ? cutTo : 'end');
 
     let cutDuration = cutTo - cutFromWithAdjustment;
     if (detectedFps != null) cutDuration = Math.max(cutDuration, frameDuration); // ensure at least one frame duration
@@ -279,7 +280,7 @@ function useFfmpegOperations({ filePath, treatInputFileModifiedTimeAsStart, trea
       ...flatMap(Object.entries(customTagsByFile[filePath] || []), ([key, value]) => ['-metadata', `${key}=${value}`]),
     ];
 
-    const mapStreamsArgs = getMapStreamsArgs({ copyFileStreams: copyFileStreamsFiltered, allFilesMeta, outFormat });
+    const mapStreamsArgs = getMapStreamsArgs({ copyFileStreams: copyFileStreamsFiltered, allFilesMeta, outFormat, areWeCutting });
 
     const customParamsArgs = (() => {
       const ret: string[] = [];

src/renderer/src/util/streams.ts

@@ -169,6 +169,8 @@ function getPerStreamFlags({ stream, outputIndex, outFormat, manuallyCopyDisposi
       // I think DV format only supports PCM_S16LE https://github.com/FFmpeg/FFmpeg/blob/b92028346c35dad837dd1160930435d88bd838b5/libavformat/dvenc.c#L450
       addCodecArgs('pcm_s16le');
       addArgs(`-ar:${outputIndex}`, '48000'); // maybe technically not lossless?
+    } else if (outFormat === 'flac' && areWeCutting && stream.codec_name === 'flac') { // https://github.com/mifi/lossless-cut/issues/1809
+      addCodecArgs('flac'); // lossless because flac is a lossless codec
     } else {
       addCodecArgs('copy');
     }