test sweep: kernel/src/panels foundational coverage (T-91)

Two test additions for the panels subsystem:

- registry_test: setTabOrder ordering contract (unknown ids sink),
  definitionFor lookup, SlotId equality + hashCode + toString.
- drag_resize_test (new): horizontal drag adjusts sidebar slot size
  via setSize, contextPanel drag inverts the delta sign, hovered
  state flips line colour.

Coverage: kernel/src/panels/slot_id.dart 5/7 -> 7/7; registry.dart
30/42 -> ~all; drag_resize.dart 0/35 -> ~all.

Total coverage 90.99% -> 91.59%; floor bumped to 91.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

pubspec.yaml

@@ -18,7 +18,7 @@ repository: https://github.com/postmeridiem/clide
 
 # Pre-push line-coverage floor. Ratchets up only — see D-66.
 # Reading: `awk -F: '/^coverage_floor:/ {gsub(/ /,"",$2); print $2}' pubspec.yaml`.
-coverage_floor: 90
+coverage_floor: 91
 
 # Project metadata (was project.yaml, folded in per D-056).
 # version: above is the single source of truth. The Makefile reads

test/kernel/src/panels/drag_resize_test.dart

@@ -0,0 +1,122 @@
+/// Tests for the DragResizeHandle widget.
+library;
+
+import 'package:clide/extension/extension.dart';
+import 'package:clide/kernel/kernel.dart';
+import 'package:clide/kernel/src/panels/drag_resize.dart';
+import 'package:flutter/gestures.dart';
+import 'package:flutter/widgets.dart';
+import 'package:flutter_test/flutter_test.dart';
+
+import '../../../helpers/kernel_fixture.dart';
+import '../../../helpers/widget_harness.dart';
+
+void main() {
+  group('DragResizeHandle', () {
+    late KernelFixture f;
+    setUp(() async => f = await KernelFixture.create());
+    tearDown(() async => f.dispose());
+
+    testWidgets('horizontal drag adjusts the sidebar slot size', (tester) async {
+      final arr = LayoutArrangement();
+      arr.applyPreset(const LayoutPresetContribution(
+        id: 'test-preset',
+        displayName: 'Test',
+        slots: [
+          LayoutSlot(slot: Slots.sidebar, position: SlotPosition.left, defaultSize: 200),
+          LayoutSlot(slot: Slots.contextPanel, position: SlotPosition.right, defaultSize: 200),
+        ],
+      ));
+      await tester.pumpWidget(harness(
+        f,
+        Center(
+          child: SizedBox(
+            width: 40,
+            height: 200,
+            child: DragResizeHandle(
+              arrangement: arr,
+              slot: Slots.sidebar,
+              axis: Axis.horizontal,
+            ),
+          ),
+        ),
+      ));
+      await tester.pumpAndSettle();
+      final center = tester.getCenter(find.byType(DragResizeHandle));
+      final gesture = await tester.startGesture(center, kind: PointerDeviceKind.mouse);
+      await gesture.moveBy(const Offset(50, 0));
+      await tester.pump();
+      await gesture.up();
+      await tester.pump();
+      expect(arr.sizeOf(Slots.sidebar), 250);
+    });
+
+    testWidgets('contextPanel drag inverts the delta sign', (tester) async {
+      final arr = LayoutArrangement();
+      arr.applyPreset(const LayoutPresetContribution(
+        id: 'test-preset',
+        displayName: 'Test',
+        slots: [
+          LayoutSlot(slot: Slots.sidebar, position: SlotPosition.left, defaultSize: 200),
+          LayoutSlot(slot: Slots.contextPanel, position: SlotPosition.right, defaultSize: 200),
+        ],
+      ));
+      await tester.pumpWidget(harness(
+        f,
+        Center(
+          child: SizedBox(
+            width: 40,
+            height: 200,
+            child: DragResizeHandle(
+              arrangement: arr,
+              slot: Slots.contextPanel,
+              axis: Axis.horizontal,
+            ),
+          ),
+        ),
+      ));
+      await tester.pumpAndSettle();
+      final center = tester.getCenter(find.byType(DragResizeHandle));
+      final gesture = await tester.startGesture(center, kind: PointerDeviceKind.mouse);
+      await gesture.moveBy(const Offset(50, 0)); // drag right → context shrinks
+      await tester.pump();
+      await gesture.up();
+      expect(arr.sizeOf(Slots.contextPanel), 150);
+    });
+
+    testWidgets('hovered state flips the line colour without throwing', (tester) async {
+      final arr = LayoutArrangement();
+      arr.applyPreset(const LayoutPresetContribution(
+        id: 'test-preset',
+        displayName: 'Test',
+        slots: [
+          LayoutSlot(slot: Slots.sidebar, position: SlotPosition.left, defaultSize: 200),
+          LayoutSlot(slot: Slots.contextPanel, position: SlotPosition.right, defaultSize: 200),
+        ],
+      ));
+      await tester.pumpWidget(harness(
+        f,
+        Center(
+          child: SizedBox(
+            width: 40,
+            height: 200,
+            child: DragResizeHandle(
+              arrangement: arr,
+              slot: Slots.sidebar,
+              axis: Axis.horizontal,
+            ),
+          ),
+        ),
+      ));
+      await tester.pumpAndSettle();
+      // Hover over the handle.
+      final gesture = await tester.createGesture(kind: PointerDeviceKind.mouse);
+      addTearDown(gesture.removePointer);
+      await gesture.addPointer(location: tester.getCenter(find.byType(DragResizeHandle)));
+      await tester.pump();
+      // Move pointer away.
+      await gesture.moveTo(const Offset(0, 0));
+      await tester.pump();
+    });
+  });
+}

test/kernel/src/panels/registry_test.dart

@@ -72,5 +72,39 @@ void main() {
       ));
       expect(r.tabsFor(Slots.sidebar), isEmpty);
     });
+
+    test('setTabOrder sorts tabsFor by the supplied order; unknown ids sink', () {
+      final r = PanelRegistry();
+      r.registerSlot(const SlotDefinition(id: Slots.sidebar, position: SlotPosition.left));
+      r.contribute(_tab(id: 'a', slot: Slots.sidebar));
+      r.contribute(_tab(id: 'b', slot: Slots.sidebar));
+      r.contribute(_tab(id: 'c', slot: Slots.sidebar));
+      expect(r.tabsFor(Slots.sidebar).map((t) => t.id), ['a', 'b', 'c']);
+      r.setTabOrder(Slots.sidebar, ['c', 'a']);
+      expect(r.tabsFor(Slots.sidebar).map((t) => t.id), ['c', 'a', 'b']);
+    });
+
+    test('definitionFor returns the SlotDefinition by id', () {
+      final r = PanelRegistry();
+      const def = SlotDefinition(id: Slots.sidebar, position: SlotPosition.left);
+      r.registerSlot(def);
+      expect(r.definitionFor(Slots.sidebar), def);
+      expect(r.definitionFor(const SlotId('nope')), isNull);
+    });
+  });
+
+  group('SlotId', () {
+    test('equality + hashCode are value-based', () {
+      const a = SlotId('foo');
+      const b = SlotId('foo');
+      const c = SlotId('bar');
+      expect(a == b, isTrue);
+      expect(a.hashCode, b.hashCode);
+      expect(a == c, isFalse);
+    });
+
+    test('toString embeds the value', () {
+      expect(const SlotId('foo').toString(), contains('foo'));
+    });
   });
 }