MultitabPane: drag-to-reorder gesture wiring (T-84)

Each tab is wrapped in a Draggable (when allowReorder is true and the
entry itself is reorderable) and a DragTarget (always — the controller's
barrier logic decides whether the move actually happens). Drops insert
the dragged entry at the target tab's index. A 2px leading insertion
indicator highlights the active drop target.

The widget harness now wraps children in an Overlay so Draggable's
feedback can mount without each test re-wrapping. Sized by the test
view's bounds to avoid disturbing existing tests that query
find.byType(SizedBox).first.

Four widget tests cover the gesture path: drop reorders, pinned
barrier blocks, pinned tabs aren't draggable, and allowReorder=false
disables drag entirely.

Co-Authored-By: Claude <noreply@anthropic.com>

test/helpers/widget_harness.dart

@@ -4,7 +4,13 @@ import 'package:flutter/widgets.dart';
 import 'kernel_fixture.dart';
 
 /// Wraps a widget in the minimum tree a primitive needs to resolve
-/// theme + i18n: `Directionality → ClideKernel → ClideTheme → child`.
+/// theme + i18n + Overlay (for Draggable feedback / Tooltip / etc.):
+/// `Directionality → ClideKernel → ClideTheme → MediaQuery →
+/// Overlay → child`.
+///
+/// The Overlay is sized by the test view's bounds via the surrounding
+/// MediaQuery; no extra SizedBox is added so existing tests that
+/// query `find.byType(SizedBox).first` still find their target.
 Widget harness(KernelFixture fixture, Widget child) {
   return Directionality(
     textDirection: TextDirection.ltr,
@@ -14,7 +20,11 @@ Widget harness(KernelFixture fixture, Widget child) {
         controller: fixture.services.theme,
         child: MediaQuery(
           data: const MediaQueryData(),
-          child: child,
+          child: Overlay(
+            initialEntries: [
+              OverlayEntry(builder: (_) => child),
+            ],
+          ),
         ),
       ),
     ),

test/widgets/multitab_pane_test.dart

@@ -151,5 +151,96 @@ void main() {
       );
       expect(find.byKey(const ValueKey('body-a')), findsNothing);
     });
+
+    testWidgets('drag a tab onto another to reorder', (tester) async {
+      final c = MultitabController<String>(initial: [entry('a'), entry('b'), entry('c')]);
+      await tester.pumpWidget(
+        harness(f, MultitabPane<String>(controller: c, bodyBuilder: body)),
+      );
+
+      // Drag tab 'a' to where tab 'c' sits.
+      final from = tester.getCenter(find.text('a'));
+      final to = tester.getCenter(find.text('c'));
+      final gesture = await tester.startGesture(from);
+      await tester.pump(const Duration(milliseconds: 100));
+      await gesture.moveTo(to);
+      await tester.pump(const Duration(milliseconds: 100));
+      await gesture.up();
+      await tester.pumpAndSettle();
+
+      expect(c.entries.map((e) => e.id), ['b', 'c', 'a']);
+    });
+
+    testWidgets('drag respects pinned barrier', (tester) async {
+      final c = MultitabController<String>(initial: [
+        entry('p', reorderable: false),
+        entry('a'),
+        entry('b'),
+      ]);
+      await tester.pumpWidget(
+        harness(f, MultitabPane<String>(controller: c, bodyBuilder: body)),
+      );
+
+      // Try to drag 'a' before pinned 'p' — controller's barrier
+      // logic should reject and the order stays.
+      final from = tester.getCenter(find.text('a'));
+      final to = tester.getCenter(find.text('p'));
+      final gesture = await tester.startGesture(from);
+      await tester.pump(const Duration(milliseconds: 100));
+      await gesture.moveTo(to);
+      await tester.pump(const Duration(milliseconds: 100));
+      await gesture.up();
+      await tester.pumpAndSettle();
+
+      expect(c.entries.map((e) => e.id), ['p', 'a', 'b']);
+    });
+
+    testWidgets('pinned tabs are not draggable', (tester) async {
+      final c = MultitabController<String>(initial: [
+        entry('p', reorderable: false),
+        entry('a'),
+      ]);
+      await tester.pumpWidget(
+        harness(f, MultitabPane<String>(controller: c, bodyBuilder: body)),
+      );
+
+      // Attempt to drag pinned 'p' to position of 'a'.
+      final from = tester.getCenter(find.text('p'));
+      final to = tester.getCenter(find.text('a'));
+      final gesture = await tester.startGesture(from);
+      await tester.pump(const Duration(milliseconds: 100));
+      await gesture.moveTo(to);
+      await tester.pump(const Duration(milliseconds: 100));
+      await gesture.up();
+      await tester.pumpAndSettle();
+
+      // Order unchanged; pinned tab refused to be dragged.
+      expect(c.entries.map((e) => e.id), ['p', 'a']);
+    });
+
+    testWidgets('allowReorder=false disables drag entirely', (tester) async {
+      final c = MultitabController<String>(initial: [entry('a'), entry('b')]);
+      await tester.pumpWidget(
+        harness(
+          f,
+          MultitabPane<String>(
+            controller: c,
+            bodyBuilder: body,
+            allowReorder: false,
+          ),
+        ),
+      );
+
+      final from = tester.getCenter(find.text('a'));
+      final to = tester.getCenter(find.text('b'));
+      final gesture = await tester.startGesture(from);
+      await tester.pump(const Duration(milliseconds: 100));
+      await gesture.moveTo(to);
+      await tester.pump(const Duration(milliseconds: 100));
+      await gesture.up();
+      await tester.pumpAndSettle();
+
+      expect(c.entries.map((e) => e.id), ['a', 'b']);
+    });
   });
 }