diff --git a/dev/index.html b/dev/index.html
index cf04aab..67386af 100644
--- a/dev/index.html
+++ b/dev/index.html
@@ -39,7 +39,7 @@
 	const context = canvas.getContext('2d');
 
 	let format = new Metamuxer.WebMOutputFormat({ streamable: false });
-	//format = new Metamuxer.Mp4OutputFormat({ fastStart: false }); // new Metamuxer.MkvOutputFormat();// new Metamuxer.Mp4OutputFormat({ fastStart: false });
+	format = new Metamuxer.Mp4OutputFormat({ fastStart: 'fragmented' }); // new Metamuxer.MkvOutputFormat();// new Metamuxer.Mp4OutputFormat({ fastStart: false });
 	let target = new Metamuxer.BufferTarget();
 
 	/*
@@ -95,7 +95,7 @@
 
 	output.addVideoTrack(videoSource);
 	output.addAudioTrack(audioSource);
-	//output.addSubtitleTrack(subtitleSource);
+	output.addSubtitleTrack(subtitleSource);
 
 	output.start();
 
@@ -174,5 +174,5 @@ Testing... <00:17.350>One... <00:18.125>Two...
 	await output.finalize();
 
 	console.log(target);
-	download(new Blob([target.buffer]), 'test.webm');
+	download(new Blob([target.buffer]), 'test.mp4');
 </script>
\ No newline at end of file
diff --git a/src/isobmff/isobmff-muxer.ts b/src/isobmff/isobmff-muxer.ts
index 9589964..3b0d53c 100644
--- a/src/isobmff/isobmff-muxer.ts
+++ b/src/isobmff/isobmff-muxer.ts
@@ -743,12 +743,14 @@ export class IsobmffMuxer extends Muxer {
 		await this.writer.flush();
 	}
 
-	private async interleaveSamples() {
+	private async interleaveSamples(isFinalCall = false) {
 		assert(this.fastStart === 'fragmented');
 
-		for (const track of this.output._tracks) {
-			if (!track.source._closed && !this.trackDatas.some(x => x.track === track)) {
-				return; // We haven't seen a sample from this open track yet
+		if (!isFinalCall) {
+			for (const track of this.output._tracks) {
+				if (!track.source._closed && !this.trackDatas.some(x => x.track === track)) {
+					return; // We haven't seen a sample from this open track yet
+				}
 			}
 		}
 
@@ -758,7 +760,7 @@ export class IsobmffMuxer extends Muxer {
 			let minTimestamp = Infinity;
 
 			for (const trackData of this.trackDatas) {
-				if (trackData.sampleQueue.length === 0 && !trackData.track.source._closed) {
+				if (!isFinalCall && trackData.sampleQueue.length === 0 && !trackData.track.source._closed) {
 					break outer;
 				}
 
@@ -878,14 +880,7 @@ export class IsobmffMuxer extends Muxer {
 		}
 
 		if (this.fastStart === 'fragmented') {
-			for (const trackData of this.trackDatas) {
-				for (const sample of trackData.sampleQueue) {
-					await this.addSampleToTrack(trackData, sample);
-				}
-
-				this.processTimestamps(trackData);
-			}
-
+			await this.interleaveSamples(true);
 			await this.finalizeFragment(false); // Don't flush the last fragment as we will flush it with the mfra box
 		} else {
 			for (const trackData of this.trackDatas) {
diff --git a/src/matroska/matroska-muxer.ts b/src/matroska/matroska-muxer.ts
index f48580d..63812f8 100644
--- a/src/matroska/matroska-muxer.ts
+++ b/src/matroska/matroska-muxer.ts
@@ -496,10 +496,12 @@ export class MatroskaMuxer extends Muxer {
 		}
 	}
 
-	private async interleaveChunks() {
-		for (const track of this.output._tracks) {
-			if (!track.source._closed && !this.trackDatas.some(x => x.track === track)) {
-				return; // We haven't seen a sample from this open track yet
+	private async interleaveChunks(isFinalCall = false) {
+		if (!isFinalCall) {
+			for (const track of this.output._tracks) {
+				if (!track.source._closed && !this.trackDatas.some(x => x.track === track)) {
+					return; // We haven't seen a sample from this open track yet
+				}
 			}
 		}
 
@@ -509,7 +511,7 @@ export class MatroskaMuxer extends Muxer {
 			let minTimestamp = Infinity;
 
 			for (const trackData of this.trackDatas) {
-				if (trackData.chunkQueue.length === 0 && !trackData.track.source._closed) {
+				if (!isFinalCall && trackData.chunkQueue.length === 0 && !trackData.track.source._closed) {
 					break outer;
 				}
 
@@ -527,7 +529,9 @@ export class MatroskaMuxer extends Muxer {
 			this.writeBlock(trackWithMinTimestamp, chunk);
 		}
 
-		await this.writer.flush();
+		if (!isFinalCall) {
+			await this.writer.flush();
+		}
 	}
 
 	/** Due to [a bug in Chromium](https://bugs.chromium.org/p/chromium/issues/detail?id=1377842), VP9 streams often
@@ -764,11 +768,7 @@ export class MatroskaMuxer extends Muxer {
 		}
 
 		// Flush any remaining queued chunks to the file
-		for (const trackData of this.trackDatas) {
-			while (trackData.chunkQueue.length > 0) {
-				this.writeBlock(trackData, trackData.chunkQueue.shift()!);
-			}
-		}
+		await this.interleaveChunks(true);
 
 		if (!this.format._options.streamable && this.currentCluster) {
 			this.finalizeCurrentCluster();
diff --git a/todo.txt b/todo.txt
index 67bd2e9..8d00277 100644
--- a/todo.txt
+++ b/todo.txt
@@ -5,5 +5,4 @@
 - A stream source?? Or like a callback-driven source
 - onHeader, etc callbacks for Matroska
 - https://github.com/Vanilagy/mp4-muxer/issues/83 tell him it's possible now
-- Matroska projection for video rotation in muxer
-- Fix interleaving at the end in finalize!!
\ No newline at end of file
+- Matroska projection for video rotation in muxer
\ No newline at end of file