diff --git a/tests/gui/__init__.py b/tests/gui/__init__.py
new file mode 100644
index 0000000..c005824
--- /dev/null
+++ b/tests/gui/__init__.py
@@ -0,0 +1,40 @@
+from pathlib import Path
+from typing import Optional
+from unittest import mock
+
+import pytest
+from pytest import MonkeyPatch
+from pytest_mock import MockerFixture
+
+from dangerzone import util
+from dangerzone.gui import Application
+from dangerzone.gui.logic import DangerzoneGui
+from dangerzone.gui.updater import UpdaterThread
+from dangerzone.isolation_provider.dummy import Dummy
+
+
+def generate_isolated_updater(
+    tmp_path: Path,
+    monkeypatch: MonkeyPatch,
+    app_mocker: MockerFixture,
+) -> UpdaterThread:
+    """Generate an Updater class with its own settings."""
+    app = app_mocker.MagicMock()
+
+    dummy = Dummy()
+    # XXX: We can monkey-patch global state without wrapping it in a context manager, or
+    # worrying that it will leak between tests, for two reasons:
+    #
+    # 1. Parallel tests in PyTest take place in different processes.
+    # 2. The monkeypatch fixture tears down the monkey-patch after each test ends.
+    monkeypatch.setattr(util, "get_config_dir", lambda: tmp_path)
+    dangerzone = DangerzoneGui(app, isolation_provider=dummy)
+    updater = UpdaterThread(dangerzone)
+    return updater
+
+
+@pytest.fixture
+def updater(
+    tmp_path: Path, monkeypatch: MonkeyPatch, mocker: MockerFixture
+) -> UpdaterThread:
+    return generate_isolated_updater(tmp_path, monkeypatch, mocker)