fix(dashboard): restore top-level tab drop target for dashboards with content (#39423)

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

superset-frontend/src/dashboard/components/DashboardBuilder/DashboardBuilder.test.tsx

@@ -24,6 +24,7 @@ import {
   screen,
 } from 'spec/helpers/testing-library';
 import { FeatureFlag } from '@superset-ui/core';
+import { supersetTheme } from '@apache-superset/core/theme';
 import {
   OPEN_FILTER_BAR_WIDTH,
   CLOSED_FILTER_BAR_WIDTH,
@@ -489,6 +490,48 @@ test('should render ParentSize wrapper with height 100% for tabs', async () => {
   expect(tabPanels.length).toBeGreaterThan(0);
 });
 
+test('should apply min-height to the top-level tab drop target so tabs can be dropped on dashboards with content', () => {
+  (useStoredSidebarWidth as jest.Mock).mockImplementation(() => [
+    100,
+    jest.fn(),
+  ]);
+  (fetchFaveStar as jest.Mock).mockReturnValue({ type: 'mock-action' });
+  (setActiveTab as jest.Mock).mockReturnValue({ type: 'mock-action' });
+
+  const { getByTestId } = render(<DashboardBuilder />, {
+    useRedux: true,
+    store: storeWithState({
+      ...mockState,
+      dashboardLayout: undoableDashboardLayout,
+      dashboardState: { ...mockState.dashboardState, editMode: true },
+    }),
+    useDnd: true,
+    useTheme: true,
+    useRouter: true,
+  });
+
+  const headerWrapper = getByTestId('dashboard-header-wrapper');
+
+  // The Droppable inside the header should have the empty-droptarget class
+  // when there are no top-level tabs and edit mode is active. Without this
+  // class (and its associated min-height CSS rule), the drop target has zero
+  // height and users cannot drag tabs onto dashboards that already have
+  // content.
+  const droptarget = headerWrapper.querySelector('.empty-droptarget');
+  expect(droptarget).toBeInTheDocument();
+
+  // Verify the StyledHeader CSS defines a non-zero min-height for
+  // .empty-droptarget, derived from theme.sizeUnit * 4 to stay in sync
+  // with the source rule in DashboardBuilder.tsx.
+  expect(headerWrapper).toHaveStyleRule(
+    'min-height',
+    `${supersetTheme.sizeUnit * 4}px`,
+    {
+      target: '.empty-droptarget',
+    },
+  );
+});
+
 test('should maintain layout when switching between tabs', async () => {
   (useStoredSidebarWidth as jest.Mock).mockImplementation(() => [
     100,

superset-frontend/src/dashboard/components/DashboardBuilder/DashboardBuilder.tsx

@@ -100,6 +100,10 @@ const StyledHeader = styled.div<{ filterBarWidth: number }>`
     z-index: 99;
     max-width: calc(100vw - ${filterBarWidth}px);
 
+    .empty-droptarget {
+      min-height: ${theme.sizeUnit * 4}px;
+    }
+
     .empty-droptarget:before {
       position: absolute;
       content: '';

superset-frontend/src/types/emotion-jest.d.ts

@@ -0,0 +1,20 @@
+/**
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+
+/// <reference types="@emotion/jest" />