Show WhatsApp popup when clicking the message in notification panel
WhatsApp has implemented this feature on Mar 29, 2016 (v2.12.560). As a fallback for quick reply feature on Android N, WhatsApp added a (kind of) quick reply from notification for all Android version.

When a notification comes, you can expand it (i.e. by sliding down) to show the Reply button. Tapping the Reply button will bring the message as a popup, which you can also reply directly from there.

 

Image courtesy of AndroidPolice