feat: cancel button to abort running exports

ExportWorker now uses Popen instead of subprocess.run so running
ffmpeg processes can be killed on cancel. Cancel button appears
between Export and worker count spinner, enabled during export.
Clips already finished before cancel are kept in the DB.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

main.py

@@ -588,6 +588,7 @@ class ExportWorker(QThread):
     finished = pyqtSignal(str)   # emitted per completed clip
     error = pyqtSignal(str)      # error message
     all_done = pyqtSignal()      # emitted after all jobs complete
+    cancelled = pyqtSignal()     # emitted when cancel completes
 
     def __init__(self, input_path: str,
                  jobs: list[tuple[float, str, str | None, float]],
@@ -602,10 +603,24 @@ class ExportWorker(QThread):
         self._image_sequence = image_sequence
         self._max_workers = max_workers
         self._encoder = encoder
+        self._cancel = False
+        self._procs: list[subprocess.Popen] = []
+        self._procs_lock = __import__('threading').Lock()
+
+    def cancel(self) -> None:
+        self._cancel = True
+        with self._procs_lock:
+            for proc in self._procs:
+                try:
+                    proc.kill()
+                except OSError:
+                    pass
 
     def _run_one(self, start: float, output: str,
                   portrait_ratio: str | None, crop_center: float) -> str:
         """Encode a single clip. Returns output path on success, raises on error."""
+        if self._cancel:
+            raise RuntimeError("cancelled")
         if self._image_sequence:
             os.makedirs(output, exist_ok=True)
         cmd = build_ffmpeg_command(
@@ -616,9 +631,22 @@ class ExportWorker(QThread):
             image_sequence=self._image_sequence,
             encoder=self._encoder,
         )
-        result = subprocess.run(cmd, capture_output=True, text=True, timeout=120)
+        proc = subprocess.Popen(cmd, stdout=subprocess.PIPE, stderr=subprocess.PIPE)
-        if result.returncode != 0:
+        with self._procs_lock:
-            raise RuntimeError(result.stderr[-500:] if result.stderr else "ffmpeg failed")
+            self._procs.append(proc)
+        try:
+            _, stderr = proc.communicate(timeout=120)
+        except subprocess.TimeoutExpired:
+            proc.kill()
+            raise RuntimeError("ffmpeg timed out")
+        finally:
+            with self._procs_lock:
+                self._procs.remove(proc)
+        if self._cancel:
+            raise RuntimeError("cancelled")
+        if proc.returncode != 0:
+            msg = stderr.decode(errors='replace')[-500:] if stderr else "ffmpeg failed"
+            raise RuntimeError(msg)
         if self._image_sequence:
             audio_cmd = build_audio_extract_command(self._input, start, output)
             subprocess.run(audio_cmd, capture_output=True, text=True, timeout=60)
@@ -634,6 +662,10 @@ class ExportWorker(QThread):
                     for s, o, pr, cc in self._jobs
                 }
                 for fut in as_completed(futures):
+                    if self._cancel:
+                        pool.shutdown(wait=False, cancel_futures=True)
+                        self.cancelled.emit()
+                        return
                     try:
                         path = fut.result()
                         self.finished.emit(path)
@@ -641,11 +673,17 @@ class ExportWorker(QThread):
                         self.error.emit("ffmpeg not found — is it installed and on PATH?")
                         return
                     except Exception as e:
+                        if self._cancel:
+                            break
                         self.error.emit(str(e))
                         return
         except Exception as e:
+            if not self._cancel:
                 self.error.emit(str(e))
             return
+        if self._cancel:
+            self.cancelled.emit()
+        else:
             self.all_done.emit()
 
 
@@ -1942,6 +1980,11 @@ class MainWindow(QMainWindow):
         self._btn_export.setToolTip("Export clips at cursor position (E)")
         self._btn_export.clicked.connect(self._on_export)
 
+        self._btn_cancel = QPushButton("Cancel")
+        self._btn_cancel.setEnabled(False)
+        self._btn_cancel.setToolTip("Cancel running export")
+        self._btn_cancel.clicked.connect(self._on_cancel_export)
+
         self._btn_delete = QPushButton("Delete")
         self._btn_delete.setEnabled(False)
         self._btn_delete.setToolTip("Delete last export or selected marker from disk and DB")
@@ -1978,6 +2021,7 @@ class MainWindow(QMainWindow):
         transport_row.addStretch()
         transport_row.addWidget(self._lbl_next)
         transport_row.addWidget(self._btn_export)
+        transport_row.addWidget(self._btn_cancel)
         transport_row.addWidget(self._spn_workers)
         transport_row.addWidget(self._btn_delete)
 
@@ -2775,6 +2819,8 @@ class MainWindow(QMainWindow):
         self._export_worker.finished.connect(self._on_clip_done)
         self._export_worker.all_done.connect(self._on_batch_done)
         self._export_worker.error.connect(self._on_export_error)
+        self._export_worker.cancelled.connect(self._on_export_cancelled)
+        self._btn_cancel.setEnabled(True)
         self._export_worker.start()
 
     def _on_clip_done(self, path: str):
@@ -2805,6 +2851,7 @@ class MainWindow(QMainWindow):
     def _on_batch_done(self):
         """Called once after all clips in the batch are done."""
         _log("Batch complete")
+        self._btn_cancel.setEnabled(False)
         self._export_counter += 1
         self._update_next_label()
         self._btn_export.setEnabled(True)
@@ -2827,12 +2874,31 @@ class MainWindow(QMainWindow):
 
     def _on_export_error(self, msg: str):
         _log(f"Export error: {msg}")
+        self._btn_cancel.setEnabled(False)
         self._btn_export.setEnabled(True)
         self._btn_export.setText("Export")
         self._btn_export.setStyleSheet("")
         self._refresh_markers()  # remove stale pending marker
         self.statusBar().showMessage(f"Export error: {msg}")
 
+    def _on_cancel_export(self):
+        if self._export_worker and self._export_worker.isRunning():
+            self._btn_cancel.setEnabled(False)
+            self._export_worker.cancel()
+            self.statusBar().showMessage("Cancelling export…")
+
+    def _on_export_cancelled(self):
+        _log("Export cancelled")
+        self._btn_export.setEnabled(True)
+        self._btn_export.setText("Export")
+        self._btn_export.setStyleSheet("")
+        self._update_next_label()
+        self._refresh_markers()
+        markers = self._db.get_markers(os.path.basename(self._file_path), self._profile)
+        if markers:
+            self._playlist.mark_done(self._file_path, len(markers))
+        self.statusBar().showMessage("Export cancelled", 4000)
+
     def changeEvent(self, event):
         super().changeEvent(event)
         if event.type() == event.Type.ActivationChange and self.isActiveWindow():