fix: use ffmpeg -progress pipe for reliable apply progress

apply_sync.py

@@ -136,6 +136,8 @@ def main():
     ap.add_argument("--b", default=None, help="Video file B (delayed audio source)")
     ap.add_argument("--lag", type=float, required=True, help="Lag in seconds (B relative to A, positive = B delayed)")
     ap.add_argument("--out", default=None, help="Output file path (overrides config)")
+    ap.add_argument("--progress", action="store_true",
+                    help="Stream ffmpeg -progress key=value lines to stdout for GUI parsing")
     args = ap.parse_args()
 
     file_a = args.a
@@ -176,11 +178,33 @@ def main():
     print(f"Output: {out_path}")
 
     cmd = build_ffmpeg_cmd(file_a, file_b, args.lag, out_path)
-    print(f"\nRunning: {' '.join(cmd)}")
-    r = subprocess.run(cmd)
-    if r.returncode != 0:
-        print(f"ERROR: ffmpeg failed with code {r.returncode}", file=sys.stderr)
-        sys.exit(r.returncode)
+    print(f"\nRunning: {' '.join(cmd)}", file=sys.stderr, flush=True)
+    if args.progress:
+        # Machine-readable progress on stdout: frame=, out_time_ms=, progress=end.
+        # -nostats suppresses the \r human stats that GUIs cannot parse line-wise.
+        # Output options must precede the output path (last element).
+        cmd = cmd[:-1] + ["-progress", "pipe:1", "-nostats", cmd[-1]]
+        # stderr stays attached to the terminal: ffmpeg's final summary line
+        # (video:/audio: sizes) must remain visible, only stats are silenced.
+        proc = subprocess.Popen(cmd, stdout=subprocess.PIPE,
+                                universal_newlines=True, bufsize=1)
+        assert proc.stdout is not None
+        for line in proc.stdout:
+            line = line.strip()
+            if not line:
+                continue
+            # Forward only progress protocol lines; keep stdout parseable.
+            if "=" in line:
+                print(line, flush=True)
+        rc = proc.wait()
+        if rc != 0:
+            print(f"ERROR: ffmpeg failed with code {rc}", file=sys.stderr)
+            sys.exit(rc)
+    else:
+        r = subprocess.run(cmd)
+        if r.returncode != 0:
+            print(f"ERROR: ffmpeg failed with code {r.returncode}", file=sys.stderr)
+            sys.exit(r.returncode)
 
     print(f"\nSynced output: {out_path}")