fix(vue): eslint/tsconfig migration + resolve type errors

@robonen/vue (toolkit): migrate to eslint flat config + composite tsconfig;
fix composable + test type errors (writable computed returns, null guards,
overload-compatible signatures, typed test helpers) — all type-level.

vue/toolkit/src/composables/browser/useDocumentVisibility/index.test.ts

@@ -0,0 +1,131 @@
+import { afterEach, describe, expect, it, vi } from 'vitest';
+import { effectScope, nextTick } from 'vue';
+import { useDocumentVisibility } from '.';
+
+afterEach(() => {
+  vi.unstubAllGlobals();
+  Object.defineProperty(document, 'visibilityState', { value: 'visible', configurable: true });
+});
+
+function setVisibility(state: DocumentVisibilityState) {
+  Object.defineProperty(document, 'visibilityState', { value: state, configurable: true });
+  document.dispatchEvent(new Event('visibilitychange'));
+}
+
+describe(useDocumentVisibility, () => {
+  it('reads the current visibility state', () => {
+    const scope = effectScope();
+    let visibility: ReturnType<typeof useDocumentVisibility>;
+    scope.run(() => {
+      visibility = useDocumentVisibility();
+    });
+
+    expect(visibility!.value).toBe('visible');
+    scope.stop();
+  });
+
+  it('updates on visibilitychange', async () => {
+    const scope = effectScope();
+    let visibility: ReturnType<typeof useDocumentVisibility>;
+    scope.run(() => {
+      visibility = useDocumentVisibility();
+    });
+
+    setVisibility('hidden');
+    await nextTick();
+
+    expect(visibility!.value).toBe('hidden');
+    scope.stop();
+  });
+
+  it('invokes onChange with new state, previous state, and the event', async () => {
+    const onChange = vi.fn();
+    const scope = effectScope();
+    scope.run(() => {
+      useDocumentVisibility({ onChange });
+    });
+
+    setVisibility('hidden');
+    await nextTick();
+
+    expect(onChange).toHaveBeenCalledTimes(1);
+    const [state, previous, event] = onChange.mock.calls[0]!;
+    expect(state).toBe('hidden');
+    expect(previous).toBe('visible');
+    expect(event).toBeInstanceOf(Event);
+
+    setVisibility('visible');
+    await nextTick();
+
+    expect(onChange).toHaveBeenCalledTimes(2);
+    expect(onChange.mock.calls[1]!.slice(0, 2)).toEqual(['visible', 'hidden']);
+    scope.stop();
+  });
+
+  it('does not update or fire onChange when the state is unchanged', async () => {
+    const onChange = vi.fn();
+    const scope = effectScope();
+    let visibility: ReturnType<typeof useDocumentVisibility>;
+    scope.run(() => {
+      visibility = useDocumentVisibility({ onChange });
+    });
+
+    // visibilityState is already 'visible'; dispatching with no real change is a no-op
+    document.dispatchEvent(new Event('visibilitychange'));
+    await nextTick();
+
+    expect(onChange).not.toHaveBeenCalled();
+    expect(visibility!.value).toBe('visible');
+    scope.stop();
+  });
+
+  it('reflects a non-default initial state at setup time', () => {
+    Object.defineProperty(document, 'visibilityState', { value: 'hidden', configurable: true });
+
+    const scope = effectScope();
+    let visibility: ReturnType<typeof useDocumentVisibility>;
+    scope.run(() => {
+      visibility = useDocumentVisibility();
+    });
+
+    expect(visibility!.value).toBe('hidden');
+    scope.stop();
+  });
+
+  it('is SSR-safe and returns "visible" without a document', () => {
+    const scope = effectScope();
+    let visibility: ReturnType<typeof useDocumentVisibility>;
+    scope.run(() => {
+      visibility = useDocumentVisibility({ document: undefined });
+    });
+
+    expect(visibility!.value).toBe('visible');
+    scope.stop();
+  });
+
+  it('accepts a custom document instance', async () => {
+    const onChange = vi.fn();
+    let listener: ((event: Event) => void) | undefined;
+    const customDoc = {
+      visibilityState: 'visible' as DocumentVisibilityState,
+      addEventListener: (_type: string, cb: (event: Event) => void) => { listener = cb; },
+      removeEventListener: vi.fn(),
+    } as unknown as Document;
+
+    const scope = effectScope();
+    let visibility: ReturnType<typeof useDocumentVisibility>;
+    scope.run(() => {
+      visibility = useDocumentVisibility({ document: customDoc, onChange });
+    });
+
+    expect(visibility!.value).toBe('visible');
+
+    (customDoc as { visibilityState: DocumentVisibilityState }).visibilityState = 'hidden';
+    listener?.(new Event('visibilitychange'));
+    await nextTick();
+
+    expect(visibility!.value).toBe('hidden');
+    expect(onChange).toHaveBeenCalledWith('hidden', 'visible', expect.any(Event));
+    scope.stop();
+  });
+});