test(studio): lock sidebar destination Y-stability on rail expansion

Adds a widget test asserting each destination icon keeps its Y position
whether the rail is collapsed or expanded — a recurring regression. To
drive expansion without a hover gesture (which trips RenderFlex overflow
mid-transition), a test-only startSidebarExpanded flag threads
StudioApp -> StudioShell -> _Sidebar (controller starts at 1.0, hover
disabled); _SidebarItems get stable ValueKeys. analyze clean; new test
and the existing smoke test pass.

Signed-off-by: flemming-it <stefan.a.flemming@googlemail.com>

lib/main.dart

@@ -60,11 +60,18 @@ class StudioApp extends StatefulWidget {
   /// ChainTheme.
   final String? initialThemePlugin;
 
+  /// Test-only: start the sidebar expanded (no hover). Lets a widget
+  /// test assert destination-icon Y stability across collapsed vs
+  /// expanded without a hover gesture (which trips RenderFlex overflow
+  /// mid-transition). Always false in production.
+  final bool startSidebarExpanded;
+
   const StudioApp({
     super.key,
     required this.initialThemeMode,
     required this.initialLocale,
     this.initialThemePlugin,
+    this.startSidebarExpanded = false,
   });
 
   /// Lookup helper so descendants can flip the theme without
@@ -226,7 +233,9 @@ class StudioAppState extends State<StudioApp> {
                     supportedLocales: AppLocalizations.supportedLocales,
                     localizationsDelegates:
                         AppLocalizations.localizationsDelegates,
-                    home: const StudioShell(),
+                    home: StudioShell(
+                      startSidebarExpanded: widget.startSidebarExpanded,
+                    ),
                   );
                 },
               ),
@@ -237,7 +246,10 @@ class StudioAppState extends State<StudioApp> {
 }
 
 class StudioShell extends StatefulWidget {
-  const StudioShell({super.key});
+  /// Test-only: forward to the sidebar so it starts expanded.
+  final bool startSidebarExpanded;
+
+  const StudioShell({super.key, this.startSidebarExpanded = false});
 
   @override
   State<StudioShell> createState() => StudioShellState();
@@ -523,6 +535,7 @@ class StudioShellState extends State<StudioShell> {
                   endpointLabel: HubService.instance.endpointLabel,
                   activeChannel: _activeChannel,
                   pendingApprovals: _pendingApprovals,
+                  forceExpanded: widget.startSidebarExpanded,
                 ),
                 Container(width: 1, color: theme.colorScheme.outlineVariant),
                 Expanded(
@@ -649,6 +662,11 @@ class _Sidebar extends StatefulWidget {
   /// without polling the page.
   final int pendingApprovals;
 
+  /// Test-only: start fully expanded (controller at 1.0) and disable
+  /// hover, so a widget test can measure the expanded layout without a
+  /// hover gesture. Always false in production.
+  final bool forceExpanded;
+
   const _Sidebar({
     required this.selectedIndex,
     required this.onSelect,
@@ -657,6 +675,7 @@ class _Sidebar extends StatefulWidget {
     required this.endpointLabel,
     required this.activeChannel,
     this.pendingApprovals = 0,
+    this.forceExpanded = false,
   });
 
   @override
@@ -694,7 +713,7 @@ class _SidebarState extends State<_Sidebar>
       // the rail feels crisp rather than sluggish.
       duration: ChainMotion.slow,
       reverseDuration: ChainMotion.base,
-      value: 0,
+      value: widget.forceExpanded ? 1 : 0,
     );
   }
 
@@ -740,8 +759,8 @@ class _SidebarState extends State<_Sidebar>
     final hasChannel =
         widget.activeChannel != null && widget.activeChannel!.isNotEmpty;
     return MouseRegion(
-      onEnter: (_) => _ctrl.forward(),
+      onEnter: widget.forceExpanded ? null : (_) => _ctrl.forward(),
-      onExit: (_) => _ctrl.reverse(),
+      onExit: widget.forceExpanded ? null : (_) => _ctrl.reverse(),
       child: AnimatedBuilder(
         animation: _ctrl,
         builder: (context, _) {
@@ -837,6 +856,7 @@ class _SidebarState extends State<_Sidebar>
                       children: [
                         for (var i = 0; i < widget.pages.length; i++)
                           _SidebarItem(
+                            key: ValueKey('sidebar-item-${widget.pages[i].id}'),
                             page: widget.pages[i],
                             selected: i == widget.selectedIndex,
                             t: t,
@@ -1342,6 +1362,7 @@ class _SidebarItem extends StatefulWidget {
   final int? badge;
 
   const _SidebarItem({
+    super.key,
     required this.page,
     required this.selected,
     required this.t,

test/sidebar_test.dart

@@ -0,0 +1,77 @@
+// Locks the sidebar invariant: destination icons sit at the same Y
+// whether the rail is collapsed or expanded. The header rows above the
+// destinations list have FIXED pixel heights (see _SidebarState), so
+// expanding the rail (a width-only change) must not shift any
+// destination vertically. This was a recurring regression source; this
+// test pins it.
+//
+// Expansion is driven via StudioApp(startSidebarExpanded: true), NOT a
+// hover gesture — hover mid-transition trips RenderFlex overflow in
+// widget tests (see widget_test.dart). With the flag the rail's
+// animation controller starts at value 1.0 (fully expanded), so no
+// animation needs to settle.
+
+import 'package:flutter/widgets.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:chain_studio/data/hub.dart';
+import 'package:chain_studio/main.dart';
+
+const _destinations = <String>[
+  'welcome',
+  'store',
+  'doctor',
+  'flows',
+  'audit',
+  'approvals',
+  'federation',
+];
+
+Map<String, double> _destinationYs(WidgetTester tester) {
+  final ys = <String, double>{};
+  for (final id in _destinations) {
+    final finder = find.byKey(ValueKey('sidebar-item-$id'));
+    expect(finder, findsOneWidget, reason: 'destination "$id" must render');
+    ys[id] = tester.getTopLeft(finder).dy;
+  }
+  return ys;
+}
+
+void main() {
+  testWidgets(
+    'sidebar destination Y positions are stable across rail expansion',
+    (tester) async {
+      // Collapsed (default).
+      await tester.pumpWidget(
+        const StudioApp(
+          initialThemeMode: ThemeModeValue.system,
+          initialLocale: Locale('en'),
+        ),
+      );
+      // pump (not pumpAndSettle): the app has long-lived timers/animations
+      // that never "settle"; one frame is enough to lay the sidebar out.
+      await tester.pump(const Duration(milliseconds: 100));
+      final collapsed = _destinationYs(tester);
+
+      // Expanded: controller starts at 1.0 via the flag, hover disabled.
+      await tester.pumpWidget(
+        const StudioApp(
+          initialThemeMode: ThemeModeValue.system,
+          initialLocale: Locale('en'),
+          startSidebarExpanded: true,
+        ),
+      );
+      await tester.pump(const Duration(milliseconds: 100));
+      final expanded = _destinationYs(tester);
+
+      for (final id in _destinations) {
+        expect(
+          expanded[id],
+          closeTo(collapsed[id]!, 0.5),
+          reason: 'destination "$id" Y shifted on expansion '
+              '(${collapsed[id]} -> ${expanded[id]}) — the rows above the '
+              'destinations list must keep fixed heights',
+        );
+      }
+    },
+  );
+}