Build the full UI (build order step 4)

main.py wires up PlayerWidget with transport controls (play/pause,
frame-step buttons + ,/. shortcuts, seek slider), mute toggle, mark
video/audio buttons, live offset readout with manual override, and an
async export flow (QThread worker so ffmpeg doesn't block the UI) that
surfaces ffmpeg's stderr log without treating stream-copy warnings as
failures.

Verified two ways, not just by reading the code: a headless smoke test
that drives the real MainWindow/ExportWorker classes through a full
mark -> offset -> export cycle on the synthetic fixture, and a live
run (actual process, real X11 window, xdotool + import screenshot)
confirming the embedded mpv surface actually renders video inside the
Qt window rather than staying blank.

scripts/smoke_test_ui.py

@@ -0,0 +1,88 @@
+"""Launches the real MainWindow, loads the out-of-sync fixture, drives the
+actual marker -> offset -> export flow through the same code the UI buttons
+call (not a copy of the logic), and grabs screenshots so it can be visually
+checked instead of just trusting the app "should" work.
+"""
+import sys
+import time
+from pathlib import Path
+
+from PyQt6.QtWidgets import QApplication
+
+sys.path.insert(0, str(Path(__file__).resolve().parent.parent))
+import main as app_main
+
+ROOT = Path(__file__).resolve().parent.parent
+CLIP = str(ROOT / "test_media" / "sync_test_OUTOFSYNC_audio+350ms.mp4")
+OUT_DIR = ROOT / "test_media" / "seek_validation"
+EXPORT_OUT = str(OUT_DIR / "_ui_smoke_export.mp4")
+
+
+def main():
+    OUT_DIR.mkdir(exist_ok=True)
+    qapp = QApplication(sys.argv)
+
+    win = app_main.MainWindow()
+    win.show()
+    qapp.processEvents()
+
+    win._load(CLIP)
+    qapp.processEvents()
+    time.sleep(0.3)
+    qapp.processEvents()
+    print(f"loaded: duration slider range = {win.seek_slider.maximum()}ms")
+
+    win.player.seek(5.000)
+    qapp.processEvents()
+    win._mark_video()
+    print(f"video mark: {win.video_mark:.3f}s -> label: {win.video_mark_label.text()}")
+
+    win.player.seek(5.350)
+    qapp.processEvents()
+    win._mark_audio()
+    print(f"audio mark: {win.audio_mark:.3f}s -> label: {win.audio_mark_label.text()}")
+
+    print(f"offset label: {win.offset_label.text()}")
+    print(f"export button enabled: {win.export_button.isEnabled()}")
+    assert win.export_button.isEnabled()
+    # hr-seek lands on the nearest frame boundary (33.3ms steps @ 30fps), so
+    # marks won't hit 5.350 exactly -- check within one frame, not bit-exact.
+    assert abs(win.offset.delay_ms - (-350.0)) < 34
+    assert win.offset.target == "video"
+
+    win.grab().save(str(OUT_DIR / "ui_smoke_after_marking.png"))
+
+    # Drive export directly through the worker (skip the file-save dialog,
+    # which needs a real user) but through the same ExportWorker class the
+    # button uses.
+    done = {"ok": False, "err": None}
+
+    def on_ok(result):
+        done["ok"] = True
+        print("export finished OK, ffmpeg log:", result.stderr[:200])
+
+    def on_err(msg, stderr):
+        done["err"] = (msg, stderr)
+
+    worker = app_main.ExportWorker(win.input_path, win.offset.delay_ms, EXPORT_OUT)
+    worker.finished_ok.connect(on_ok)
+    worker.finished_err.connect(on_err)
+    worker.start()
+
+    deadline = time.time() + 15
+    while not done["ok"] and done["err"] is None and time.time() < deadline:
+        qapp.processEvents()
+        time.sleep(0.05)
+
+    if done["err"]:
+        raise RuntimeError(f"export failed: {done['err']}")
+    assert done["ok"], "export did not finish within timeout"
+    assert Path(EXPORT_OUT).exists()
+    print(f"exported file exists: {EXPORT_OUT}, size={Path(EXPORT_OUT).stat().st_size} bytes")
+
+    win.player.shutdown()
+    print("\nUI SMOKE TEST PASSED")
+
+
+if __name__ == "__main__":
+    main()