fix

lib/app/design_system/mw_authentication.dart

@@ -311,9 +311,7 @@ class _Tab extends StatelessWidget {
         alignment: Alignment.center,
         decoration: BoxDecoration(
           border: Border(
-            bottom: BorderSide(
+            bottom: BorderSide(color: selected ? accent : Colors.transparent),
-              color: selected ? accent : const Color(0x247FB8D4),
-            ),
           ),
         ),
         child: Text(

test/app/design_system/mw_authentication_test.dart

@@ -81,4 +81,40 @@ void main() {
 
     expect(requestedMode, MwAuthenticationMode.registration);
   });
+
+  testWidgets('inactive authentication tab has no divider', (tester) async {
+    await tester.pumpWidget(
+      MaterialApp(
+        theme: MoonWellTheme.create(MoonWellThemeVariant.forest),
+        home: Scaffold(
+          body: MwAuthenticationForm(
+            usernameController: TextEditingController(),
+            passwordController: TextEditingController(),
+            onSubmit: () {},
+          ),
+        ),
+      ),
+    );
+
+    final tabBorders = tester
+        .widgetList<Container>(
+          find.descendant(
+            of: find.byType(MwAuthenticationForm),
+            matching: find.byType(Container),
+          ),
+        )
+        .map((container) => container.decoration)
+        .whereType<BoxDecoration>()
+        .map((decoration) => decoration.border)
+        .whereType<Border>()
+        .where((border) => border.bottom.style != BorderStyle.none)
+        .map((border) => border.bottom.color)
+        .toList();
+
+    expect(tabBorders, hasLength(2));
+    expect(
+      tabBorders.where((color) => color == Colors.transparent),
+      hasLength(1),
+    );
+  });
 }