feat: poll server data so shared pages stay near-live

Refetch the current page every 15s while the tab is visible, gated by a
pure refresh policy: edit pages, hidden or offline tabs and in-flight
navigations are left alone. Replaces refresh-on-refocus with auto-refresh.

2 files changed, 74 insertions, 0 deletions

diff --git a/src/lib/shared/refresh-policy.test.ts b/src/lib/shared/refresh-policy.test.ts
new file mode 100644
index 0000000..5cab234
--- /dev/null
+++ b/src/lib/shared/refresh-policy.test.ts
@@ -0,0 +1,43 @@
+import { describe, it, expect } from 'vitest';
+import { shouldAutoRefresh } from './refresh-policy';
+
+const idle = { visible: true, online: true, navigating: false };
+
+describe('shouldAutoRefresh', () => {
+	it('refreshes a list page when the tab is visible and idle', () => {
+		expect(shouldAutoRefresh({ routeId: '/groceries', ...idle })).toBe(true);
+		expect(shouldAutoRefresh({ routeId: '/groceries/catalog', ...idle })).toBe(
+			true,
+		);
+		expect(shouldAutoRefresh({ routeId: '/projects/[id]', ...idle })).toBe(
+			true,
+		);
+	});
+
+	it('never refreshes a page holding an in-progress edit', () => {
+		expect(shouldAutoRefresh({ routeId: '/grocery/[id]', ...idle })).toBe(
+			false,
+		);
+		expect(
+			shouldAutoRefresh({
+				routeId: '/projects/[projectId]/routine/[routineId]',
+				...idle,
+			}),
+		).toBe(false);
+		expect(shouldAutoRefresh({ routeId: '/login', ...idle })).toBe(false);
+	});
+
+	it('holds off while hidden, offline or mid-navigation', () => {
+		const routeId = '/groceries';
+		expect(shouldAutoRefresh({ routeId, ...idle, visible: false })).toBe(false);
+		expect(shouldAutoRefresh({ routeId, ...idle, online: false })).toBe(false);
+		expect(shouldAutoRefresh({ routeId, ...idle, navigating: true })).toBe(
+			false,
+		);
+	});
+
+	it('holds off when the route is not resolved yet', () => {
+		expect(shouldAutoRefresh({ routeId: null, ...idle })).toBe(false);
+		expect(shouldAutoRefresh({ routeId: undefined, ...idle })).toBe(false);
+	});
+});
diff --git a/src/lib/shared/refresh-policy.ts b/src/lib/shared/refresh-policy.ts
new file mode 100644
index 0000000..d79e6b7
--- /dev/null
+++ b/src/lib/shared/refresh-policy.ts
@@ -0,0 +1,31 @@
+/**
+ * Routes whose page data must not be refetched in the background: edit pages
+ * hold in-progress work, and refreshing the login form is pointless.
+ */
+export const EXCLUDED_ROUTES: Record<string, true> = {
+	'/login': true,
+	'/grocery/[id]': true,
+	'/projects/[projectId]/routine/[routineId]': true,
+};
+
+export interface RefreshConditions {
+	/** `$page.route.id`; null when no route matched yet. */
+	routeId: string | null | undefined;
+	/** Document is visible (not a backgrounded tab). */
+	visible: boolean;
+	/** Browser reports a network connection. */
+	online: boolean;
+	/** A navigation or form action is already in flight. */
+	navigating: boolean;
+}
+
+/** A background refetch may re-run the page load only if nothing is at stake. */
+export function shouldAutoRefresh({
+	routeId,
+	visible,
+	online,
+	navigating,
+}: RefreshConditions): boolean {
+	if (!routeId || routeId in EXCLUDED_ROUTES) return false;
+	return visible && online && !navigating;
+}