Allow dynamically import atmosphere.js for SW context

Fixes #2867

packages/ts/frontend/src/FluxConnection.ts

@@ -1,5 +1,7 @@
 import type { ReactiveControllerHost } from '@lit/reactive-element';
-import atmosphere from 'atmosphere.js';
+
+import type Atmosphere from 'atmosphere.js';
+
 import type { Subscription } from './Connect.js';
 import { getCsrfTokenHeadersForEndpointRequest } from './CsrfUtils.js';
 import {
@@ -9,6 +11,8 @@ import {
   type ServerMessage,
 } from './FluxMessages.js';
 
+let atmosphere: Atmosphere.Atmosphere;
+
 export enum State {
   ACTIVE = 'active',
   INACTIVE = 'inactive',
@@ -71,6 +75,14 @@ type EndpointInfo = {
   reconnect?(): ActionOnLostSubscription | void;
 };
 
+interface ImportMetaEnv {
+  readonly SW_CONTEXT: boolean;
+}
+
+interface ImportMeta {
+  readonly env: ImportMetaEnv;
+}
+
 /**
  * A representation of the underlying persistent network connection used for subscribing to Flux type endpoint methods.
  */
@@ -89,7 +101,17 @@ export class FluxConnection extends EventTarget {
 
   constructor(connectPrefix: string, atmosphereOptions?: Partial<Atmosphere.Request>) {
     super();
-    this.#connectWebsocket(connectPrefix.replace(/connect$/u, ''), atmosphereOptions ?? {});
+    // @ts-expect-error - vite environment variable
+    const meta: ImportMeta = import.meta;
+    if (!meta.env.SW_CONTEXT) {
+      (async () => {
+        atmosphere = await import('atmosphere.js');
+        this.#connectWebsocket(connectPrefix.replace(/connect$/u, ''), atmosphereOptions ?? {});
+      })().catch((e) => {
+        // eslint-disable-next-line no-console
+        console.error('Failed to load atmosphere.js', e);
+      });
+    }
   }
 
   #resubscribeIfWasClosed() {