When pwa is in background, the serviceworker still uses onMessage not onBackgroundMessage!

Hello,

when I send a push notification to my app, the onMessage method is called even if it is in the background! Actually, the onBackgroundMessage should then be called, right? The problem now is that when I tap on the message box in Android, the Progressive Web Application is not brought to the foreground. Apparently Android thinks that the app is in the foreground? The message box disappears when I tap on it and the number of incoming messages also disappears from the app icon.

Is there any way to fix this?

The onMessage is defined in a service
`this.message$ = new Observable(sub => onMessage(this.messaging, it => sub.next(it))).pipe(
tap(payload => {this.print.info('MessagingService initFCMMessaging() FCM fmessage', { payload });})
);`

The onBackgroundMessage is defined in the firebase-messaging-sw.js with static text.

`messaging.onBackgroundMessage(function(payload) {
// Customize notification here
const notificationTitle = 'My title';
const notificationOptions = {
body: 'My Bodytext',
icon: '/firebase-logo.png' || '/assets/icons/icon-72x72.png'
};

self.registration.showNotification(notificationTitle, notificationOptions);
});`

I only noticed this because I defined a static text for the background message.

先比較 service 的 onMessage 設定與 firebase-messaging-sw.js 中的 onBackgroundMessage 註冊，然後在 PWA 於背景執行時重現該行為。當預期的 handler 已建立，且通知點擊行為已修正或限制已清楚記錄時，即表示完成。

由索引模型根據 Issue 內容生成。