diff --git a/tests/test_plugin_preferences_regressions.py b/tests/test_plugin_preferences_regressions.py
new file mode 100644
index 0000000..3b67415
--- /dev/null
+++ b/tests/test_plugin_preferences_regressions.py
@@ -0,0 +1,82 @@
+#!/usr/bin/env python3
+
+"""Regression tests for the plugin controls in Preferences."""
+
+import unittest
+from unittest import mock
+
+from cthulhu import cthulhu  # Import first to satisfy the application's initialization order.
+from cthulhu import cthulhu_gui_prefs
+
+
+class PluginPreferencesRegressionTests(unittest.TestCase):
+    """Keep plugin state changes independent from their presentation widgets."""
+
+    @staticmethod
+    def _create_gui():
+        gui = cthulhu_gui_prefs.CthulhuSetupGUI.__new__(
+            cthulhu_gui_prefs.CthulhuSetupGUI
+        )
+        gui.prefsDict = {
+            "activePlugins": ["HiddenPlugin", "SourceOne"],
+            "pluginSources": ["https://example.com/plugins.git"],
+        }
+        gui._available_plugins = {"HiddenPlugin", "SourceOne", "SourceTwo", "Required"}
+        gui._plugin_entries = {
+            "SourceOne": {"active": True, "can_toggle": True},
+            "SourceTwo": {"active": False, "can_toggle": True},
+            "Required": {"active": True, "can_toggle": False},
+        }
+        gui._plugin_canonical_map = {
+            "SourceOne": "SharedPlugin",
+            "SourceTwo": "SharedPlugin",
+            "Required": "Required",
+        }
+        gui._plugin_group_map = {
+            "SharedPlugin": ["SourceOne", "SourceTwo"],
+            "Required": ["Required"],
+        }
+        gui._update_selected_plugin_controls = mock.Mock()
+        gui._update_plugin_tabs = mock.Mock()
+        return gui
+
+    def test_active_plugins_include_hidden_and_selected_plugins(self):
+        gui = self._create_gui()
+
+        self.assertEqual(
+            gui._get_active_plugins_from_ui(),
+            ["HiddenPlugin", "SourceOne", "Required"],
+        )
+
+    def test_enabling_plugin_disables_other_copy_with_same_name(self):
+        gui = self._create_gui()
+
+        gui._set_plugin_active("SourceTwo", True)
+
+        self.assertFalse(gui._plugin_entries["SourceOne"]["active"])
+        self.assertTrue(gui._plugin_entries["SourceTwo"]["active"])
+        gui._update_selected_plugin_controls.assert_called_once_with()
+        gui._update_plugin_tabs.assert_called_once_with()
+
+    def test_required_plugin_cannot_be_disabled(self):
+        gui = self._create_gui()
+
+        gui._set_plugin_active("Required", False)
+
+        self.assertTrue(gui._plugin_entries["Required"]["active"])
+        gui._update_plugin_tabs.assert_not_called()
+
+    def test_apply_does_not_modify_plugin_sources(self):
+        gui = self._create_gui()
+        gui._populate_plugin_list = mock.Mock()
+
+        gui._apply_plugin_changes()
+
+        self.assertEqual(
+            gui.prefsDict["pluginSources"],
+            ["https://example.com/plugins.git"],
+        )
+
+
+if __name__ == "__main__":
+    unittest.main()