diff --git a/src/requirements.txt b/src/requirements.txt
index c0e094c..0c335d9 100644
--- a/src/requirements.txt
+++ b/src/requirements.txt
@@ -33,3 +33,5 @@ python-multipart
 orjson
 argon2-cffi
 toga-core>=0.5.2
+pillow
+toga-dummy>=0.5.2  # for headless GUI tests
diff --git a/src/seedpass_gui/__init__.py b/src/seedpass_gui/__init__.py
index f473abc..4ef96bc 100644
--- a/src/seedpass_gui/__init__.py
+++ b/src/seedpass_gui/__init__.py
@@ -1,11 +1,11 @@
 """Graphical user interface for SeedPass."""
 
-from .app import SeedPassApp
+from .app import SeedPassApp, build
 
 
 def main() -> None:
     """Launch the GUI application."""
-    SeedPassApp().main_loop()
+    build().main_loop()
 
 
 __all__ = ["SeedPassApp", "main"]
diff --git a/src/seedpass_gui/app.py b/src/seedpass_gui/app.py
index b3b84cb..f1578f1 100644
--- a/src/seedpass_gui/app.py
+++ b/src/seedpass_gui/app.py
@@ -16,10 +16,12 @@ class LockScreenWindow(toga.Window):
     """Window prompting for the master password."""
 
     def __init__(
-        self, app: SeedPassApp, vault: VaultService, entries: EntryService
+        self, controller: SeedPassApp, vault: VaultService, entries: EntryService
     ) -> None:
         super().__init__("Unlock Vault")
-        self.app = app
+        # Store a reference to the SeedPass application instance separately from
+        # the ``toga`` ``Window.app`` attribute to avoid conflicts.
+        self.controller = controller
         self.vault = vault
         self.entries = entries
 
@@ -43,8 +45,8 @@ class LockScreenWindow(toga.Window):
         except Exception as exc:  # pragma: no cover - GUI error handling
             self.message.text = str(exc)
             return
-        main = MainWindow(self.app, self.vault, self.entries)
-        self.app.main_window = main
+        main = MainWindow(self.controller, self.vault, self.entries)
+        self.controller.main_window = main
         main.show()
         self.close()
 
@@ -53,10 +55,12 @@ class MainWindow(toga.Window):
     """Main application window showing vault entries."""
 
     def __init__(
-        self, app: SeedPassApp, vault: VaultService, entries: EntryService
+        self, controller: SeedPassApp, vault: VaultService, entries: EntryService
     ) -> None:
         super().__init__("SeedPass")
-        self.app = app
+        # ``Window.app`` is reserved for the Toga ``App`` instance. Store the
+        # SeedPass application reference separately.
+        self.controller = controller
         self.vault = vault
         self.entries = entries
 
@@ -115,7 +119,7 @@ class EntryDialog(toga.Window):
         self.username_input = toga.TextInput(style=Pack(flex=1))
         self.url_input = toga.TextInput(style=Pack(flex=1))
         self.length_input = toga.NumberInput(
-            min_value=8, max_value=128, style=Pack(width=80), value=16
+            min=8, max=128, style=Pack(width=80), value=16
         )
 
         save_button = toga.Button(
@@ -184,7 +188,8 @@ class SearchDialog(toga.Window):
 
 
 def build() -> SeedPassApp:
-    return SeedPassApp()
+    """Return a configured :class:`SeedPassApp` instance."""
+    return SeedPassApp(formal_name="SeedPass", app_id="org.seedpass.gui")
 
 
 class SeedPassApp(toga.App):
@@ -201,4 +206,4 @@ class SeedPassApp(toga.App):
 
 def main() -> None:  # pragma: no cover - GUI bootstrap
     """Run the BeeWare application."""
-    SeedPassApp().main_loop()
+    build().main_loop()
diff --git a/src/tests/test_gui_headless.py b/src/tests/test_gui_headless.py
new file mode 100644
index 0000000..67b5557
--- /dev/null
+++ b/src/tests/test_gui_headless.py
@@ -0,0 +1,69 @@
+import os
+from types import SimpleNamespace
+
+import toga
+
+from seedpass_gui.app import LockScreenWindow, MainWindow, EntryDialog
+
+
+class FakeVault:
+    def __init__(self):
+        self.called = False
+
+    def unlock(self, request):
+        self.called = True
+
+
+class FakeEntries:
+    def __init__(self):
+        self.added = []
+        self.modified = []
+
+    def list_entries(self):
+        return []
+
+    def search_entries(self, query):
+        return []
+
+    def add_entry(self, label, length, username=None, url=None):
+        self.added.append((label, length, username, url))
+        return 1
+
+    def modify_entry(self, entry_id, username=None, url=None, label=None):
+        self.modified.append((entry_id, username, url, label))
+
+
+def setup_module(module):
+    os.environ["TOGA_BACKEND"] = "toga_dummy"
+    import asyncio
+
+    asyncio.set_event_loop(asyncio.new_event_loop())
+
+
+def test_unlock_creates_main_window():
+    app = toga.App("Test", "org.example")
+    controller = SimpleNamespace(main_window=None)
+    vault = FakeVault()
+    entries = FakeEntries()
+
+    win = LockScreenWindow(controller, vault, entries)
+    win.password_input.value = "pw"
+    win.handle_unlock(None)
+
+    assert vault.called
+    assert isinstance(controller.main_window, MainWindow)
+
+
+def test_entrydialog_add_calls_service():
+    toga.App("Test2", "org.example2")
+    entries = FakeEntries()
+    main = SimpleNamespace(entries=entries, refresh_entries=lambda: None)
+
+    dlg = EntryDialog(main, None)
+    dlg.label_input.value = "L"
+    dlg.username_input.value = "u"
+    dlg.url_input.value = "x"
+    dlg.length_input.value = 12
+    dlg.save(None)
+
+    assert entries.added == [("L", 12, "u", "x")]