frontend/src/components/DraggableDroppableTagRow.test.tsx

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+import { beforeEach, describe, expect, it, vi } from 'vitest'
+
+import DraggableDroppableTagRow from '@/components/DraggableDroppableTagRow'
+import { buildTag } from '@/test/factories'
+import { renderWithProviders } from '@/test/render'
+
+const dndKit = vi.hoisted (() => ({
+  useDraggable: vi.fn (),
+  useDroppable: vi.fn (),
+}))
+
+vi.mock ('@dnd-kit/core', () => dndKit)
+
+const tag = buildTag ({ id: 7, name: 'ドラッグ元', postCount: 3 })
+
+const renderRow = (activeDndId?: string) => {
+  renderWithProviders (
+      <DraggableDroppableTagRow
+          activeDndId={activeDndId}
+          tag={tag}
+          nestLevel={2}
+          pathKey="cat-general-7"
+          suppressClickRef={{ current: false }}/>,
+  )
+}
+
+const tagBody = (): HTMLElement => {
+  const link = document.querySelector<HTMLElement> ('a[title="ドラッグ元"]')
+
+  if (link == null)
+    throw new Error ('tag link not found')
+
+  const body = link.closest ('div')
+
+  if (body == null)
+    throw new Error ('tag body not found')
+
+  return body
+}
+
+describe ('DraggableDroppableTagRow', () => {
+  beforeEach (() => {
+    vi.clearAllMocks ()
+    dndKit.useDraggable.mockReturnValue ({
+        attributes: { 'aria-describedby': 'drag-source' },
+        listeners:  { onPointerDown: vi.fn () },
+        setNodeRef: vi.fn (),
+        transform:  null })
+    dndKit.useDroppable.mockReturnValue ({
+        isOver:     false,
+        setNodeRef: vi.fn () })
+  })
+
+  it ('passes dndId through draggable data for active-row tracking', () => {
+    renderRow ()
+
+    expect (dndKit.useDraggable).toHaveBeenCalledWith (
+        expect.objectContaining ({
+            id:   'tag-node:cat-general-7',
+            data: expect.objectContaining ({
+                dndId:     'tag-node:cat-general-7',
+                nestLevel: 2,
+                tagId:     7 }) }))
+  })
+
+  it ('hides only the active drag source from the explicit active dnd id', () => {
+    renderRow ('tag-node:cat-general-7')
+    expect (tagBody ()).toHaveStyle ({ visibility: 'hidden' })
+  })
+
+  it ('keeps inactive tag rows visible while another tag is dragged', () => {
+    renderRow ('tag-node:other')
+    expect (tagBody ()).toHaveStyle ({ visibility: 'visible' })
+  })
+})