Fetch sender certificates on-demand

ts/util/waitForOnline.ts

@@ -0,0 +1,21 @@
+// Copyright 2021 Signal Messenger, LLC
+// SPDX-License-Identifier: AGPL-3.0-only
+
+export function waitForOnline(
+  navigator: Readonly<{ onLine: boolean }>,
+  onlineEventTarget: EventTarget
+): Promise<void> {
+  return new Promise(resolve => {
+    if (navigator.onLine) {
+      resolve();
+      return;
+    }
+
+    const listener = () => {
+      onlineEventTarget.removeEventListener('online', listener);
+      resolve();
+    };
+
+    onlineEventTarget.addEventListener('online', listener);
+  });
+}