App for WhatsApp™ Web is the easiest way to open and check WhatsApp from your desktop machine (PC, MAC and Linux). Clicking on the launch button, renders WhatsApp Web in a standalone UI. Moreover, the window is re-sizable to your desired choice and all the changes are auto saved to the memory.
App for WhatsApp™ Web is designed to operate similar to Google Chrome Apps. In other words, it opens in a simple and stand-alone popup window (as opposed to the toolbar-popup or sidebar), and once closed, completely clears-off and won't have any further background activities. Having this behavior, it is considered a very lite Addon and does not add any additional burden to your Opera browser. Moreover, the popup has a built-in "Always-On-Top" feature which makes the window remain always on top of other windows, and enables you to do multi-tasking.
Important Note: App for WhatsApp™ Web does NOT belong or related to the official WhatsApp application in any ways. It is an unofficial app that is developed and maintained independently. The official WhatsApp software is only released for smart phones and the web. This unofficial app (browser addon) is a wrapper to the original WhatsApp web application and tries to make it usable as a browser plug-in.
When you open the popup, you will see a toolbar with 5 buttons at the top. Toolbar buttons are described in the screenshot below (2nd picture). Note, after few seconds the toolbar will disappear by itself. But when you move the mouse to the top area, it will appear again.
Note: to start working with WhatsApp Web, you can read this FAQ page in WhatsApp website (https://www.whatsapp.com/faq/en/web/28080003).