Initial

tests/unit/persisted.test.ts

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+import { describe, it, expect, beforeEach, vi } from 'vitest';
+import { get } from 'svelte/store';
+import { persisted } from '$state';
+
+beforeEach(() => localStorage.clear());
+
+describe('persisted', () => {
+	it('uses the initial value when storage is empty', () => {
+		const s = persisted('k1', 42);
+		expect(get(s)).toBe(42);
+	});
+	it('writes updates to localStorage', () => {
+		const s = persisted('k2', { n: 1 });
+		s.set({ n: 2 });
+		expect(JSON.parse(localStorage.getItem('k2')!)).toEqual({ n: 2 });
+	});
+	it('rehydrates a value written by a previous instance', () => {
+		persisted('k3', 'a').set('b');
+		const s2 = persisted('k3', 'a');
+		expect(get(s2)).toBe('b');
+	});
+
+	/**
+	 * Regression: constructing a store used to broadcast the value it had just
+	 * read, so opening a second tab pushed its stale snapshot over the first one.
+	 */
+	it('does not broadcast the value read at construction', () => {
+		const posted: unknown[] = [];
+		class SpyChannel {
+			onmessage: ((e: { data: unknown }) => void) | null = null;
+			postMessage(v: unknown) {
+				posted.push(v);
+			}
+		}
+		vi.stubGlobal('BroadcastChannel', SpyChannel);
+
+		localStorage.setItem('k4', JSON.stringify({ n: 1 }));
+		const s = persisted('k4', { n: 0 });
+		expect(get(s)).toEqual({ n: 1 });
+		expect(posted).toEqual([]); // nothing announced merely by mounting
+
+		s.set({ n: 2 });
+		expect(posted).toEqual([{ n: 2 }]); // local changes still propagate
+
+		vi.unstubAllGlobals();
+	});
+});