WhatsApp Desktop for the Mac supports the picture-in-picture with the newly released version 0.3.1850. Image playback of videos. The app is available for free in the Mac App Store.
Picture-in-Picture means in this case that you can start playback in a separate window by clicking on the corresponding icon. The new video window then hovers over the WhatsApp window, you can move the window and switch to other parts of the app while looking at it. However, it is not possible to place the picture-in-picture window outside the WhatsApp application, for example, to use another app while viewing.
WhatsApp Desktop for the Mac is basically the WhatsApp web view packaged in an app. To use it, the connection to a version of WhatsApp active on a mobile phone is required.