According to the selected users, “the desktop client is set to be always on the top so you will never lose your video chats whenever you change your tab or window”
WhatsApp confirmed that the video call feature will work “seamlessly” for both portrait and landscape orientation. In addition, WhatsApp also needs access to your microphone and your webcam for your voice and video calling. In order to make a voice or video call on the desktop version of WhatsApp, you have to make sure that you are connected to the internet on your computer.
If you don’t have one you can download it from the official WhatsApp site.
How to video call using WhatsApp Desktop app?