diff --git a/stores/product-workspace/screen-state.ts b/stores/product-workspace/screen-state.ts
new file mode 100644
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--- /dev/null
+++ b/stores/product-workspace/screen-state.ts
@@ -0,0 +1,33 @@
+// Expliciete schermstaat voor de Product Backlog page.
+//
+// Consolideert de vandaag verspreide schermstaat-afleiding (page.tsx,
+// sprint-switcher.tsx, new-sprint-trigger.tsx, save-sprint-button.tsx) tot één
+// pure, testbare functie. Zie docs/architecture/product-backlog-workflow.md,
+// sectie "To-be: expliciete state machine".
+//
+// PRODUCT_NOT_ACTIVE en DEMO_MODE blijven bewust BUITEN ScreenState — het zijn
+// cross-cutting gates, geen knopen in de state machine.
+
+export type ScreenState =
+  | { kind: 'NO_SPRINT' }
+  | { kind: 'DRAFT' }
+  | { kind: 'ACTIVE'; building: boolean }
+  | { kind: 'EDITING'; building: boolean }
+
+export interface ScreenStateInput {
+  activeSprintItem: { id: string } | null // SSR-prop uit page.tsx
+  buildingSprintIds: string[] // SSR-prop uit page.tsx
+  hasPendingDraft: boolean // user-settings store
+  pendingAdds: string[] // product-workspace store: sprintMembership.pending.adds
+  pendingRemoves: string[] // product-workspace store: sprintMembership.pending.removes
+}
+
+export function deriveScreenState(i: ScreenStateInput): ScreenState {
+  if (i.hasPendingDraft) return { kind: 'DRAFT' } // draft wint van alles
+  if (i.activeSprintItem) {
+    const building = i.buildingSprintIds.includes(i.activeSprintItem.id)
+    const dirty = i.pendingAdds.length > 0 || i.pendingRemoves.length > 0
+    return dirty ? { kind: 'EDITING', building } : { kind: 'ACTIVE', building }
+  }
+  return { kind: 'NO_SPRINT' }
+}