diff --git a/src/renderer/stores/preview-store.ts b/src/renderer/stores/preview-store.ts
new file mode 100644
index 0000000..250c601
--- /dev/null
+++ b/src/renderer/stores/preview-store.ts
@@ -0,0 +1,25 @@
+import { create } from 'zustand';
+
+interface PreviewState {
+  source: string;
+  scrollRatio: number;
+  setSource: (s: string) => void;
+  setScrollRatio: (r: number) => void;
+}
+
+const DEBOUNCE_MS = 300;
+let timer: ReturnType<typeof setTimeout> | null = null;
+let pending: string = '';
+
+export const usePreviewStore = create<PreviewState>((set) => ({
+  source: '',
+  scrollRatio: 0,
+  setSource: (s) => {
+    pending = s;
+    if (timer) clearTimeout(timer);
+    timer = setTimeout(() => {
+      set({ source: pending });
+    }, DEBOUNCE_MS);
+  },
+  setScrollRatio: (r) => set({ scrollRatio: r }),
+}));
diff --git a/tests/unit/stores/preview-store.test.ts b/tests/unit/stores/preview-store.test.ts
new file mode 100644
index 0000000..92cd380
--- /dev/null
+++ b/tests/unit/stores/preview-store.test.ts
@@ -0,0 +1,23 @@
+import { describe, it, expect, beforeEach, vi } from 'vitest';
+import { usePreviewStore } from '@/stores/preview-store';
+
+describe('usePreviewStore', () => {
+  beforeEach(() => {
+    vi.useFakeTimers();
+    usePreviewStore.setState({ html: '', scrollRatio: 0, source: '' });
+  });
+
+  it('debounces source updates (300 ms)', () => {
+    usePreviewStore.getState().setSource('# a');
+    usePreviewStore.getState().setSource('# b');
+    usePreviewStore.getState().setSource('# c');
+    expect(usePreviewStore.getState().source).toBe('');
+    vi.advanceTimersByTime(300);
+    expect(usePreviewStore.getState().source).toBe('# c');
+  });
+
+  it('updates scroll ratio', () => {
+    usePreviewStore.getState().setScrollRatio(0.5);
+    expect(usePreviewStore.getState().scrollRatio).toBe(0.5);
+  });
+});