diff --git a/__tests__/stores/product-workspace/screen-state.test.ts b/__tests__/stores/product-workspace/screen-state.test.ts
new file mode 100644
index 0000000..7463fff
--- /dev/null
+++ b/__tests__/stores/product-workspace/screen-state.test.ts
@@ -0,0 +1,83 @@
+import { describe, it, expect } from 'vitest'
+import {
+  deriveScreenState,
+  type ScreenStateInput,
+} from '@/stores/product-workspace/screen-state'
+
+const base: ScreenStateInput = {
+  activeSprintItem: null,
+  buildingSprintIds: [],
+  hasPendingDraft: false,
+  pendingAdds: [],
+  pendingRemoves: [],
+}
+
+describe('deriveScreenState', () => {
+  it('returns NO_SPRINT without draft or active sprint', () => {
+    expect(deriveScreenState(base)).toEqual({ kind: 'NO_SPRINT' })
+  })
+
+  it('returns DRAFT when a pending draft exists', () => {
+    expect(deriveScreenState({ ...base, hasPendingDraft: true })).toEqual({
+      kind: 'DRAFT',
+    })
+  })
+
+  it('lets a draft win over an active sprint with pending changes', () => {
+    expect(
+      deriveScreenState({
+        ...base,
+        hasPendingDraft: true,
+        activeSprintItem: { id: 's1' },
+        pendingAdds: ['x'],
+      }),
+    ).toEqual({ kind: 'DRAFT' })
+  })
+
+  it('returns ACTIVE for an active sprint with no pending changes', () => {
+    expect(
+      deriveScreenState({ ...base, activeSprintItem: { id: 's1' } }),
+    ).toEqual({ kind: 'ACTIVE', building: false })
+  })
+
+  it('flags building when the active sprint is in buildingSprintIds', () => {
+    expect(
+      deriveScreenState({
+        ...base,
+        activeSprintItem: { id: 's1' },
+        buildingSprintIds: ['s1'],
+      }),
+    ).toEqual({ kind: 'ACTIVE', building: true })
+  })
+
+  it('returns EDITING when there are pending adds', () => {
+    expect(
+      deriveScreenState({
+        ...base,
+        activeSprintItem: { id: 's1' },
+        pendingAdds: ['x'],
+      }),
+    ).toEqual({ kind: 'EDITING', building: false })
+  })
+
+  it('returns EDITING when there are pending removes', () => {
+    expect(
+      deriveScreenState({
+        ...base,
+        activeSprintItem: { id: 's1' },
+        pendingRemoves: ['y'],
+      }),
+    ).toEqual({ kind: 'EDITING', building: false })
+  })
+
+  it('flags building on EDITING when the active sprint is building', () => {
+    expect(
+      deriveScreenState({
+        ...base,
+        activeSprintItem: { id: 's1' },
+        pendingAdds: ['x'],
+        buildingSprintIds: ['s1'],
+      }),
+    ).toEqual({ kind: 'EDITING', building: true })
+  })
+})