Initial commit

.env.example

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+NTFY_URL=
+NTFY_TOPIC=
+NTFY_TOKEN=
+

.gitignore

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+.env
+.codex-home/
+.venv/
+__pycache__/
+*.py[cod]
+.pytest_cache/
+dist/
+build/
+*.spec

README.md

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+# Stall Watcher
+
+Stall Watcher is a small passive Windows desktop app that monitors a selected screen region and sends a notification when that region stays visually still for a configured duration.
+
+It only uses local screenshots and image comparison. It does not click, type, inject code, read another process memory, or interact with the watched application.
+
+## Install
+
+Requires Python 3.12+.
+
+```bash
+python -m venv .venv
+.venv\Scripts\activate
+pip install -r requirements.txt
+```
+
+## Configure ntfy
+
+Copy `.env.example` to `.env` and fill in your own values:
+
+```env
+NTFY_URL=https://your.ntfy.server
+NTFY_TOPIC=stall-watcher
+NTFY_TOKEN=your-token
+```
+
+The `.env` file is only for ntfy secrets. App preferences are stored separately with Qt settings.
+
+## Run
+
+From the repository root:
+
+```bash
+python -m src.main
+```
+
+## Use
+
+1. Select the monitor to watch.
+2. Click **Select region**.
+3. Drag a rectangle over the area to monitor. Press Escape to cancel.
+4. Adjust capture interval, alert delay, and movement threshold.
+5. Enable Windows and/or ntfy notifications.
+6. Click **Start monitoring**.
+
+The live motion score helps calibrate the threshold. For example, if motion is around `8.4` while moving and `0.7` while stopped, a threshold around `2.0` is a reasonable starting point.
+
+## Build a Windows exe
+
+Install PyInstaller in your virtualenv:
+
+```bash
+pip install pyinstaller
+pyinstaller --noconsole --name StallWatcher --paths . src/main.py
+```
+
+The executable will be created under `dist/`.
+
+## Development
+
+Run tests:
+
+```bash
+pytest
+```
+
+The detector tests cover motion scoring, still-time accumulation, reset on movement, one alert per still period, and rearming after movement resumes.
+

requirements.txt

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+PySide6>=6.7
+mss>=9.0
+numpy>=1.26
+python-dotenv>=1.0
+requests>=2.31
+pytest>=8.0
+

src/__init__.py

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+

src/capture.py

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+from __future__ import annotations
+
+from typing import Iterable
+
+import mss
+import numpy as np
+
+from .models import MonitorInfo, Region
+
+
+class CaptureError(RuntimeError):
+    pass
+
+
+def list_monitors() -> list[MonitorInfo]:
+    try:
+        with mss.mss() as sct:
+            return [
+                MonitorInfo(
+                    id=index,
+                    left=int(monitor["left"]),
+                    top=int(monitor["top"]),
+                    width=int(monitor["width"]),
+                    height=int(monitor["height"]),
+                )
+                for index, monitor in enumerate(sct.monitors[1:], start=1)
+            ]
+    except Exception as exc:
+        raise CaptureError(f"Unable to list monitors: {exc}") from exc
+
+
+def get_monitor(monitors: Iterable[MonitorInfo], monitor_id: int) -> MonitorInfo | None:
+    return next((monitor for monitor in monitors if monitor.id == monitor_id), None)
+
+
+def validate_region(region: Region, monitor: MonitorInfo) -> None:
+    if not region.is_valid:
+        raise CaptureError("Selected region is empty.")
+    if region.monitor_id != monitor.id:
+        raise CaptureError("Selected region belongs to a disconnected or different monitor.")
+    if region.x < monitor.left or region.y < monitor.top:
+        raise CaptureError("Selected region starts outside the monitor.")
+    if region.x + region.width > monitor.left + monitor.width:
+        raise CaptureError("Selected region is wider than the monitor bounds.")
+    if region.y + region.height > monitor.top + monitor.height:
+        raise CaptureError("Selected region is taller than the monitor bounds.")
+
+
+def capture_region(region: Region) -> np.ndarray:
+    bbox = {
+        "left": region.x,
+        "top": region.y,
+        "width": region.width,
+        "height": region.height,
+    }
+    try:
+        with mss.mss() as sct:
+            shot = sct.grab(bbox)
+            return np.asarray(shot, dtype=np.uint8)[:, :, :3]
+    except Exception as exc:
+        raise CaptureError(f"Unable to capture selected region: {exc}") from exc
+

src/config.py

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+from __future__ import annotations
+
+import os
+from dataclasses import dataclass
+
+from dotenv import load_dotenv
+from PySide6.QtCore import QSettings
+
+from .models import DetectionConfig, Region
+
+
+ORG_NAME = "StallWatcher"
+APP_NAME = "Stall Watcher"
+
+
+@dataclass(frozen=True)
+class NtfyConfig:
+    url: str
+    topic: str
+    token: str
+
+    @property
+    def is_complete(self) -> bool:
+        return bool(self.url and self.topic and self.token)
+
+
+def load_ntfy_config() -> NtfyConfig:
+    load_dotenv()
+    return NtfyConfig(
+        url=os.getenv("NTFY_URL", "").rstrip("/"),
+        topic=os.getenv("NTFY_TOPIC", "").strip("/"),
+        token=os.getenv("NTFY_TOKEN", ""),
+    )
+
+
+class AppSettings:
+    def __init__(self) -> None:
+        self._settings = QSettings(ORG_NAME, APP_NAME)
+
+    def monitor_id(self) -> int:
+        return int(self._settings.value("monitor_id", 1))
+
+    def set_monitor_id(self, monitor_id: int) -> None:
+        self._settings.setValue("monitor_id", int(monitor_id))
+
+    def region(self) -> Region | None:
+        width = int(self._settings.value("region/width", 0))
+        height = int(self._settings.value("region/height", 0))
+        if width <= 0 or height <= 0:
+            return None
+        return Region(
+            x=int(self._settings.value("region/x", 0)),
+            y=int(self._settings.value("region/y", 0)),
+            width=width,
+            height=height,
+            monitor_id=int(self._settings.value("region/monitor_id", self.monitor_id())),
+        )
+
+    def set_region(self, region: Region) -> None:
+        self._settings.setValue("region/x", region.x)
+        self._settings.setValue("region/y", region.y)
+        self._settings.setValue("region/width", region.width)
+        self._settings.setValue("region/height", region.height)
+        self._settings.setValue("region/monitor_id", region.monitor_id)
+
+    def detection_config(self) -> DetectionConfig:
+        return DetectionConfig(
+            movement_threshold=float(self._settings.value("movement_threshold", 2.0)),
+            alert_after=float(self._settings.value("alert_after", 15.0)),
+            capture_interval=float(self._settings.value("capture_interval", 1.0)),
+        )
+
+    def set_detection_config(self, config: DetectionConfig) -> None:
+        self._settings.setValue("movement_threshold", config.movement_threshold)
+        self._settings.setValue("alert_after", config.alert_after)
+        self._settings.setValue("capture_interval", config.capture_interval)
+
+    def enable_ntfy(self) -> bool:
+        return self._as_bool(self._settings.value("enable_ntfy", True))
+
+    def set_enable_ntfy(self, enabled: bool) -> None:
+        self._settings.setValue("enable_ntfy", enabled)
+
+    def enable_windows_notifications(self) -> bool:
+        return self._as_bool(self._settings.value("enable_windows_notifications", True))
+
+    def set_enable_windows_notifications(self, enabled: bool) -> None:
+        self._settings.setValue("enable_windows_notifications", enabled)
+
+    @staticmethod
+    def _as_bool(value: object) -> bool:
+        if isinstance(value, bool):
+            return value
+        return str(value).lower() in {"1", "true", "yes"}
+

src/detector.py

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+from __future__ import annotations
+
+import numpy as np
+
+from .models import DetectionConfig, DetectionState, DetectionStatus
+
+
+class MotionDetector:
+    def __init__(self, config: DetectionConfig, downscale_step: int = 2) -> None:
+        self.config = config
+        self.downscale_step = max(1, downscale_step)
+        self.previous_frame: np.ndarray | None = None
+        self.state = DetectionState.MOVING
+        self.still_time = 0.0
+
+    def reset(self) -> None:
+        self.previous_frame = None
+        self.state = DetectionState.MOVING
+        self.still_time = 0.0
+
+    def update(self, frame: np.ndarray, elapsed: float | None = None) -> DetectionStatus:
+        current = self._prepare_frame(frame)
+        if self.previous_frame is None:
+            self.previous_frame = current
+            return DetectionStatus(0.0, self.still_time, self.state, False)
+
+        score = self.motion_score_from_prepared(self.previous_frame, current)
+        self.previous_frame = current
+        interval = self.config.capture_interval if elapsed is None else elapsed
+        return self._advance_state(score, interval)
+
+    def _advance_state(self, motion_score: float, elapsed: float) -> DetectionStatus:
+        moving = motion_score >= self.config.movement_threshold
+        should_alert = False
+
+        if self.state == DetectionState.MOVING:
+            if moving:
+                self.still_time = 0.0
+            else:
+                self.still_time = elapsed
+                self.state = DetectionState.STILL_WAITING
+
+        elif self.state == DetectionState.STILL_WAITING:
+            if moving:
+                self.still_time = 0.0
+                self.state = DetectionState.MOVING
+            else:
+                self.still_time += elapsed
+                if self.still_time >= self.config.alert_after:
+                    should_alert = True
+                    self.state = DetectionState.ALERTED
+
+        elif self.state == DetectionState.ALERTED:
+            if moving:
+                self.still_time = 0.0
+                self.state = DetectionState.MOVING
+
+        return DetectionStatus(motion_score, self.still_time, self.state, should_alert)
+
+    def calculate_motion_score(self, previous: np.ndarray, current: np.ndarray) -> float:
+        return self.motion_score_from_prepared(
+            self._prepare_frame(previous),
+            self._prepare_frame(current),
+        )
+
+    def _prepare_frame(self, frame: np.ndarray) -> np.ndarray:
+        if frame.ndim == 3:
+            rgb = frame[..., :3].astype(np.float32)
+            gray = (0.299 * rgb[..., 0]) + (0.587 * rgb[..., 1]) + (0.114 * rgb[..., 2])
+        elif frame.ndim == 2:
+            gray = frame.astype(np.float32)
+        else:
+            raise ValueError("Frame must be a 2D grayscale or 3D RGB/BGR array.")
+        return gray[:: self.downscale_step, :: self.downscale_step]
+
+    @staticmethod
+    def motion_score_from_prepared(previous: np.ndarray, current: np.ndarray) -> float:
+        if previous.shape != current.shape:
+            raise ValueError("Frames must have the same shape.")
+        diff = np.abs(current.astype(np.float32) - previous.astype(np.float32))
+        return float(np.clip((diff.mean() / 255.0) * 100.0, 0.0, 100.0))
+

src/main.py

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+from __future__ import annotations
+
+from pathlib import Path
+import sys
+
+from dotenv import load_dotenv
+from PySide6.QtWidgets import QApplication
+
+if __package__ in {None, ""}:
+    sys.path.insert(0, str(Path(__file__).resolve().parents[1]))
+    from src.config import APP_NAME, ORG_NAME
+    from src.ui.main_window import MainWindow
+else:
+    from .config import APP_NAME, ORG_NAME
+    from .ui.main_window import MainWindow
+
+
+def main() -> int:
+    load_dotenv()
+    app = QApplication(sys.argv)
+    app.setOrganizationName(ORG_NAME)
+    app.setApplicationName(APP_NAME)
+    window = MainWindow(app)
+    window.show()
+    return app.exec()
+
+
+if __name__ == "__main__":
+    raise SystemExit(main())

src/models.py

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+from __future__ import annotations
+
+from dataclasses import dataclass
+from enum import Enum
+
+
+class DetectionState(str, Enum):
+    MOVING = "MOVING"
+    STILL_WAITING = "STILL_WAITING"
+    ALERTED = "ALERTED"
+
+
+class UiStatus(str, Enum):
+    STOPPED = "STOPPED"
+    MOVING = "MOVING"
+    WAITING = "WAITING"
+    ALERTED = "ALERTED"
+
+
+@dataclass(frozen=True)
+class MonitorInfo:
+    id: int
+    left: int
+    top: int
+    width: int
+    height: int
+
+    @property
+    def label(self) -> str:
+        return f"Monitor {self.id} - {self.width}x{self.height}"
+
+
+@dataclass(frozen=True)
+class Region:
+    x: int
+    y: int
+    width: int
+    height: int
+    monitor_id: int
+
+    @property
+    def is_valid(self) -> bool:
+        return self.width > 0 and self.height > 0
+
+
+@dataclass(frozen=True)
+class DetectionConfig:
+    movement_threshold: float = 2.0
+    alert_after: float = 15.0
+    capture_interval: float = 1.0
+
+
+@dataclass(frozen=True)
+class DetectionStatus:
+    motion_score: float
+    still_time: float
+    state: DetectionState
+    should_alert: bool = False
+

src/notifications/__init__.py

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+

src/notifications/ntfy.py

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+from __future__ import annotations
+
+import requests
+
+from ..config import NtfyConfig
+
+
+class NtfyNotifier:
+    def __init__(self, config: NtfyConfig, timeout: float = 5.0) -> None:
+        self.config = config
+        self.timeout = timeout
+
+    def send(self, message: str, title: str = "Stall Watcher") -> tuple[bool, str]:
+        if not self.config.is_complete:
+            return False, "ntfy is not configured."
+
+        url = f"{self.config.url}/{self.config.topic}"
+        headers = {
+            "Authorization": f"Bearer {self.config.token}",
+            "Title": title,
+            "Priority": "high",
+        }
+        try:
+            response = requests.post(url, data=message.encode("utf-8"), headers=headers, timeout=self.timeout)
+            response.raise_for_status()
+            return True, "ntfy notification sent."
+        except requests.RequestException as exc:
+            return False, f"ntfy notification failed: {exc}"
+

src/notifications/windows.py

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+from __future__ import annotations
+
+from PySide6.QtGui import QIcon
+from PySide6.QtWidgets import QApplication, QSystemTrayIcon
+
+
+class WindowsNotifier:
+    def __init__(self, app: QApplication) -> None:
+        self._tray = QSystemTrayIcon(QIcon(), app)
+        self._tray.setToolTip("Stall Watcher")
+        self._tray.show()
+
+    def send(self, message: str, title: str = "Stall Watcher") -> tuple[bool, str]:
+        try:
+            if not QSystemTrayIcon.isSystemTrayAvailable():
+                return False, "System tray is not available."
+            self._tray.showMessage(title, message, QSystemTrayIcon.MessageIcon.Warning, 8000)
+            return True, "Windows notification sent."
+        except Exception as exc:
+            return False, f"Windows notification failed: {exc}"
+

src/ui/__init__.py

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+

src/ui/main_window.py

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+from __future__ import annotations
+
+import time
+
+from PySide6.QtCore import QObject, QThread, Signal, Slot
+from PySide6.QtWidgets import (
+    QApplication,
+    QCheckBox,
+    QComboBox,
+    QDoubleSpinBox,
+    QFormLayout,
+    QGridLayout,
+    QGroupBox,
+    QLabel,
+    QMainWindow,
+    QMessageBox,
+    QPushButton,
+    QSpinBox,
+    QVBoxLayout,
+    QWidget,
+)
+
+from ..capture import CaptureError, capture_region, get_monitor, list_monitors, validate_region
+from ..config import AppSettings, load_ntfy_config
+from ..detector import MotionDetector
+from ..models import DetectionConfig, DetectionState, DetectionStatus, MonitorInfo, Region, UiStatus
+from ..notifications.ntfy import NtfyNotifier
+from ..notifications.windows import WindowsNotifier
+from .region_selector import RegionSelector
+
+
+class MonitorWorker(QObject):
+    status_changed = Signal(object)
+    error = Signal(str)
+    alert = Signal(str)
+    finished = Signal()
+
+    def __init__(self, region: Region, config: DetectionConfig) -> None:
+        super().__init__()
+        self.region = region
+        self.config = config
+        self._running = False
+        self._detector = MotionDetector(config)
+
+    @Slot()
+    def run(self) -> None:
+        self._running = True
+        last_capture = time.monotonic()
+        try:
+            while self._running:
+                started = time.monotonic()
+                frame = capture_region(self.region)
+                elapsed = max(0.0, started - last_capture)
+                last_capture = started
+                status = self._detector.update(frame, elapsed or self.config.capture_interval)
+                self.status_changed.emit(status)
+                if status.should_alert:
+                    message = f"No movement detected for {self.config.alert_after:g} seconds."
+                    self.alert.emit(message)
+                sleep_for = max(0.05, self.config.capture_interval - (time.monotonic() - started))
+                end_time = time.monotonic() + sleep_for
+                while self._running and time.monotonic() < end_time:
+                    QThread.msleep(50)
+        except CaptureError as exc:
+            self.error.emit(str(exc))
+        except Exception as exc:
+            self.error.emit(f"Monitoring failed: {exc}")
+        finally:
+            self.finished.emit()
+
+    @Slot()
+    def stop(self) -> None:
+        self._running = False
+
+
+class MainWindow(QMainWindow):
+    def __init__(self, app: QApplication) -> None:
+        super().__init__()
+        self.setWindowTitle("Stall Watcher")
+        self.resize(440, 560)
+        self._settings = AppSettings()
+        self._monitors: list[MonitorInfo] = []
+        self._region: Region | None = self._settings.region()
+        self._selector: RegionSelector | None = None
+        self._thread: QThread | None = None
+        self._worker: MonitorWorker | None = None
+        self._windows_notifier = WindowsNotifier(app)
+        self._build_ui()
+        self._load_monitors()
+        self._load_settings()
+        self._refresh_region_label()
+        self._set_status(UiStatus.STOPPED)
+
+    def closeEvent(self, event) -> None:  # type: ignore[no-untyped-def]
+        self._save_settings()
+        self.stop_monitoring()
+        super().closeEvent(event)
+
+    def _build_ui(self) -> None:
+        root = QWidget()
+        layout = QVBoxLayout(root)
+
+        title = QLabel("Stall Watcher")
+        title.setStyleSheet("font-size: 22px; font-weight: 600;")
+        layout.addWidget(title)
+
+        monitoring_group = QGroupBox("Monitoring")
+        monitoring_layout = QFormLayout(monitoring_group)
+        self.monitor_combo = QComboBox()
+        self.monitor_combo.currentIndexChanged.connect(self._monitor_changed)
+        self.select_region_button = QPushButton("Select region")
+        self.select_region_button.clicked.connect(self._select_region)
+        self.region_label = QLabel("No region selected")
+        monitoring_layout.addRow("Monitor:", self.monitor_combo)
+        monitoring_layout.addRow("Region:", self.select_region_button)
+        monitoring_layout.addRow("Selected region:", self.region_label)
+        layout.addWidget(monitoring_group)
+
+        detection_group = QGroupBox("Detection")
+        detection_layout = QFormLayout(detection_group)
+        self.capture_interval = QDoubleSpinBox()
+        self.capture_interval.setRange(0.1, 60.0)
+        self.capture_interval.setSingleStep(0.1)
+        self.capture_interval.setSuffix(" seconds")
+        self.alert_after = QSpinBox()
+        self.alert_after.setRange(1, 3600)
+        self.alert_after.setSuffix(" seconds")
+        self.threshold = QDoubleSpinBox()
+        self.threshold.setRange(0.0, 100.0)
+        self.threshold.setSingleStep(0.1)
+        self.threshold.setDecimals(2)
+        self.motion_score_label = QLabel("0.00")
+        self.still_for_label = QLabel("0.0 s")
+        self.state_label = QLabel("STOPPED")
+        self.status_dot = QLabel()
+        self.status_dot.setFixedSize(16, 16)
+        status_row = QWidget()
+        status_layout = QGridLayout(status_row)
+        status_layout.setContentsMargins(0, 0, 0, 0)
+        status_layout.addWidget(self.status_dot, 0, 0)
+        status_layout.addWidget(self.state_label, 0, 1)
+        detection_layout.addRow("Capture interval:", self.capture_interval)
+        detection_layout.addRow("Alert after:", self.alert_after)
+        detection_layout.addRow("Movement threshold:", self.threshold)
+        detection_layout.addRow("Motion score:", self.motion_score_label)
+        detection_layout.addRow("Still for:", self.still_for_label)
+        detection_layout.addRow("State:", status_row)
+        layout.addWidget(detection_group)
+
+        notifications_group = QGroupBox("Notifications")
+        notifications_layout = QFormLayout(notifications_group)
+        self.windows_checkbox = QCheckBox("Windows notification")
+        self.ntfy_checkbox = QCheckBox("ntfy")
+        self.test_button = QPushButton("Test notification")
+        self.test_button.clicked.connect(self._test_notification)
+        self.notification_result = QLabel("")
+        notifications_layout.addRow(self.windows_checkbox)
+        notifications_layout.addRow(self.ntfy_checkbox)
+        notifications_layout.addRow(self.test_button)
+        notifications_layout.addRow(self.notification_result)
+        layout.addWidget(notifications_group)
+
+        button_row = QWidget()
+        button_layout = QGridLayout(button_row)
+        button_layout.setContentsMargins(0, 0, 0, 0)
+        self.start_button = QPushButton("Start monitoring")
+        self.stop_button = QPushButton("Stop")
+        self.stop_button.setEnabled(False)
+        self.start_button.clicked.connect(self.start_monitoring)
+        self.stop_button.clicked.connect(self.stop_monitoring)
+        button_layout.addWidget(self.start_button, 0, 0)
+        button_layout.addWidget(self.stop_button, 0, 1)
+        layout.addWidget(button_row)
+
+        self.setCentralWidget(root)
+
+    def _load_monitors(self) -> None:
+        self.monitor_combo.clear()
+        try:
+            self._monitors = list_monitors()
+        except CaptureError as exc:
+            self._monitors = []
+            QMessageBox.warning(self, "Monitor error", str(exc))
+
+        for monitor in self._monitors:
+            self.monitor_combo.addItem(monitor.label, monitor.id)
+
+        if not self._monitors:
+            self.monitor_combo.addItem("No monitor detected", None)
+            self.start_button.setEnabled(False)
+            self.select_region_button.setEnabled(False)
+            return
+
+        saved_id = self._settings.monitor_id()
+        index = self.monitor_combo.findData(saved_id)
+        self.monitor_combo.setCurrentIndex(index if index >= 0 else 0)
+
+    def _load_settings(self) -> None:
+        config = self._settings.detection_config()
+        self.capture_interval.setValue(config.capture_interval)
+        self.alert_after.setValue(int(config.alert_after))
+        self.threshold.setValue(config.movement_threshold)
+        self.ntfy_checkbox.setChecked(self._settings.enable_ntfy())
+        self.windows_checkbox.setChecked(self._settings.enable_windows_notifications())
+
+    def _current_monitor(self) -> MonitorInfo | None:
+        monitor_id = self.monitor_combo.currentData()
+        if monitor_id is None:
+            return None
+        return get_monitor(self._monitors, int(monitor_id))
+
+    def _monitor_changed(self) -> None:
+        monitor = self._current_monitor()
+        if monitor is not None:
+            self._settings.set_monitor_id(monitor.id)
+
+    def _select_region(self) -> None:
+        monitor = self._current_monitor()
+        if monitor is None:
+            QMessageBox.warning(self, "Region", "No monitor is available.")
+            return
+        self._selector = RegionSelector(monitor)
+        self._selector.region_selected.connect(self._region_selected)
+        self._selector.selection_cancelled.connect(lambda: self.notification_result.setText("Region selection cancelled."))
+        self._selector.show()
+        self._selector.activateWindow()
+        self._selector.raise_()
+
+    def _region_selected(self, region: Region) -> None:
+        self._region = region
+        self._settings.set_region(region)
+        self._refresh_region_label()
+
+    def _refresh_region_label(self) -> None:
+        if self._region is None:
+            self.region_label.setText("No region selected")
+            return
+        self.region_label.setText(
+            f"X: {self._region.x}\nY: {self._region.y}\n"
+            f"Width: {self._region.width}\nHeight: {self._region.height}"
+        )
+
+    def _detection_config(self) -> DetectionConfig:
+        return DetectionConfig(
+            movement_threshold=float(self.threshold.value()),
+            alert_after=float(self.alert_after.value()),
+            capture_interval=float(self.capture_interval.value()),
+        )
+
+    def _save_settings(self) -> None:
+        self._settings.set_detection_config(self._detection_config())
+        self._settings.set_enable_ntfy(self.ntfy_checkbox.isChecked())
+        self._settings.set_enable_windows_notifications(self.windows_checkbox.isChecked())
+
+    def start_monitoring(self) -> None:
+        self._save_settings()
+        monitor = self._current_monitor()
+        if monitor is None:
+            QMessageBox.warning(self, "Monitoring", "No monitor is available.")
+            return
+        if self._region is None:
+            QMessageBox.warning(self, "Monitoring", "Select a region before starting.")
+            return
+        try:
+            validate_region(self._region, monitor)
+        except CaptureError as exc:
+            QMessageBox.warning(self, "Monitoring", str(exc))
+            return
+
+        self.start_button.setEnabled(False)
+        self.stop_button.setEnabled(True)
+        self._set_status(UiStatus.MOVING)
+        self._thread = QThread(self)
+        self._worker = MonitorWorker(self._region, self._detection_config())
+        self._worker.moveToThread(self._thread)
+        self._thread.started.connect(self._worker.run)
+        self._worker.status_changed.connect(self._handle_status)
+        self._worker.alert.connect(self._send_alert)
+        self._worker.error.connect(self._handle_worker_error)
+        self._worker.finished.connect(self._thread.quit)
+        self._worker.finished.connect(self._worker.deleteLater)
+        self._thread.finished.connect(self._thread.deleteLater)
+        self._thread.finished.connect(self._monitoring_finished)
+        self._thread.start()
+
+    def stop_monitoring(self) -> None:
+        if self._worker is not None:
+            self._worker.stop()
+        if self._thread is not None and self._thread.isRunning():
+            self._thread.quit()
+            self._thread.wait(2000)
+
+    def _monitoring_finished(self) -> None:
+        self._worker = None
+        self._thread = None
+        self.start_button.setEnabled(True)
+        self.stop_button.setEnabled(False)
+        self._set_status(UiStatus.STOPPED)
+
+    def _handle_status(self, status: DetectionStatus) -> None:
+        self.motion_score_label.setText(f"{status.motion_score:.2f}")
+        self.still_for_label.setText(f"{status.still_time:.1f} s")
+        if status.state == DetectionState.MOVING:
+            self._set_status(UiStatus.MOVING)
+        elif status.state == DetectionState.STILL_WAITING:
+            self._set_status(UiStatus.WAITING)
+        else:
+            self._set_status(UiStatus.ALERTED)
+
+    def _set_status(self, status: UiStatus) -> None:
+        colors = {
+            UiStatus.STOPPED: "#666666",
+            UiStatus.MOVING: "#1f9d55",
+            UiStatus.WAITING: "#d99000",
+            UiStatus.ALERTED: "#d92d20",
+        }
+        self.state_label.setText(status.value)
+        self.status_dot.setStyleSheet(f"background: {colors[status]}; border-radius: 8px;")
+
+    def _send_alert(self, message: str) -> None:
+        results = self._send_notifications(message)
+        self.notification_result.setText(" | ".join(results))
+
+    def _test_notification(self) -> None:
+        self._save_settings()
+        results = self._send_notifications("Test notification from Stall Watcher")
+        self.notification_result.setText(" | ".join(results) if results else "No notification channel enabled.")
+
+    def _send_notifications(self, message: str) -> list[str]:
+        results: list[str] = []
+        if self.windows_checkbox.isChecked():
+            ok, detail = self._windows_notifier.send(message)
+            results.append(("OK: " if ok else "Error: ") + detail)
+        if self.ntfy_checkbox.isChecked():
+            ok, detail = NtfyNotifier(load_ntfy_config()).send(message)
+            results.append(("OK: " if ok else "Error: ") + detail)
+        return results
+
+    def _handle_worker_error(self, message: str) -> None:
+        self.notification_result.setText(message)
+        QMessageBox.warning(self, "Monitoring error", message)
+        self.stop_monitoring()

src/ui/region_selector.py

@@ -0,0 +1,74 @@
+from __future__ import annotations
+
+from PySide6.QtCore import Qt, QRect, QPoint, Signal
+from PySide6.QtGui import QColor, QKeyEvent, QMouseEvent, QPainter, QPen
+from PySide6.QtWidgets import QWidget
+
+from ..models import MonitorInfo, Region
+
+
+class RegionSelector(QWidget):
+    region_selected = Signal(object)
+    selection_cancelled = Signal()
+
+    def __init__(self, monitor: MonitorInfo) -> None:
+        super().__init__()
+        self.monitor = monitor
+        self._origin: QPoint | None = None
+        self._current: QPoint | None = None
+        self.setWindowFlags(Qt.WindowType.FramelessWindowHint | Qt.WindowType.WindowStaysOnTopHint)
+        self.setAttribute(Qt.WidgetAttribute.WA_TranslucentBackground)
+        self.setGeometry(monitor.left, monitor.top, monitor.width, monitor.height)
+        self.setCursor(Qt.CursorShape.CrossCursor)
+        self.setFocusPolicy(Qt.FocusPolicy.StrongFocus)
+
+    def mousePressEvent(self, event: QMouseEvent) -> None:
+        if event.button() == Qt.MouseButton.LeftButton:
+            self._origin = event.position().toPoint()
+            self._current = self._origin
+            self.update()
+
+    def mouseMoveEvent(self, event: QMouseEvent) -> None:
+        if self._origin is not None:
+            self._current = event.position().toPoint()
+            self.update()
+
+    def mouseReleaseEvent(self, event: QMouseEvent) -> None:
+        if event.button() != Qt.MouseButton.LeftButton or self._origin is None:
+            return
+        self._current = event.position().toPoint()
+        rect = self._selection_rect()
+        if rect.width() > 2 and rect.height() > 2:
+            self.region_selected.emit(
+                Region(
+                    x=self.monitor.left + rect.x(),
+                    y=self.monitor.top + rect.y(),
+                    width=rect.width(),
+                    height=rect.height(),
+                    monitor_id=self.monitor.id,
+                )
+            )
+        else:
+            self.selection_cancelled.emit()
+        self.close()
+
+    def keyPressEvent(self, event: QKeyEvent) -> None:
+        if event.key() == Qt.Key.Key_Escape:
+            self.selection_cancelled.emit()
+            self.close()
+
+    def paintEvent(self, event) -> None:  # type: ignore[no-untyped-def]
+        painter = QPainter(self)
+        painter.fillRect(self.rect(), QColor(0, 0, 0, 90))
+        if self._origin is None or self._current is None:
+            return
+        rect = self._selection_rect()
+        painter.fillRect(rect, QColor(60, 140, 255, 55))
+        pen = QPen(QColor(60, 140, 255), 2)
+        painter.setPen(pen)
+        painter.drawRect(rect)
+
+    def _selection_rect(self) -> QRect:
+        if self._origin is None or self._current is None:
+            return QRect()
+        return QRect(self._origin, self._current).normalized()

tests/test_detector.py

@@ -0,0 +1,67 @@
+import numpy as np
+
+from src.detector import MotionDetector
+from src.models import DetectionConfig, DetectionState
+
+
+def detector() -> MotionDetector:
+    return MotionDetector(DetectionConfig(movement_threshold=2.0, alert_after=3.0, capture_interval=1.0), downscale_step=1)
+
+
+def test_identical_images_score_near_zero() -> None:
+    image = np.zeros((10, 10, 3), dtype=np.uint8)
+    assert detector().calculate_motion_score(image, image) == 0.0
+
+
+def test_different_images_score_is_higher() -> None:
+    first = np.zeros((10, 10, 3), dtype=np.uint8)
+    second = np.full((10, 10, 3), 255, dtype=np.uint8)
+    assert detector().calculate_motion_score(first, second) > 90.0
+
+
+def test_still_time_accumulates_below_threshold() -> None:
+    det = detector()
+    image = np.zeros((10, 10, 3), dtype=np.uint8)
+    det.update(image, 1.0)
+    status = det.update(image, 1.0)
+    assert status.state == DetectionState.STILL_WAITING
+    assert status.still_time == 1.0
+    status = det.update(image, 1.0)
+    assert status.still_time == 2.0
+
+
+def test_still_time_resets_when_movement_returns() -> None:
+    det = detector()
+    still = np.zeros((10, 10, 3), dtype=np.uint8)
+    moved = np.full((10, 10, 3), 255, dtype=np.uint8)
+    det.update(still, 1.0)
+    det.update(still, 1.0)
+    status = det.update(moved, 1.0)
+    assert status.state == DetectionState.MOVING
+    assert status.still_time == 0.0
+
+
+def test_only_one_alert_during_one_still_period() -> None:
+    det = detector()
+    image = np.zeros((10, 10, 3), dtype=np.uint8)
+    det.update(image, 1.0)
+    assert not det.update(image, 1.0).should_alert
+    assert not det.update(image, 1.0).should_alert
+    assert det.update(image, 1.0).should_alert
+    assert not det.update(image, 1.0).should_alert
+    assert not det.update(image, 1.0).should_alert
+    assert det.state == DetectionState.ALERTED
+
+
+def test_alert_rearms_after_movement_resumes() -> None:
+    det = detector()
+    still = np.zeros((10, 10, 3), dtype=np.uint8)
+    moved = np.full((10, 10, 3), 255, dtype=np.uint8)
+    det.update(still, 1.0)
+    det.update(still, 1.0)
+    det.update(still, 1.0)
+    assert det.update(still, 1.0).should_alert
+    assert det.update(moved, 1.0).state == DetectionState.MOVING
+    det.update(moved, 1.0)
+    det.update(moved, 1.0)
+    assert det.update(moved, 1.0).should_alert