There's again a new feature in the latest beta version of WhatsApp. A â€œReplyâ€-button in notifications, which enables you to reply even faster.
Now when you get a notification from WhatsApp, there's a quick-reply option in the notification. Choosing this option opens a pop-up which we already know from WhatsApp. But now you haven't to enable the pop-up feature to reply on WhatsApp from any app, without opening WhatsApp itself.
Here are some screenshots how this looks like:
How you can sign-up for the beta version of WhatsApp is described here.