The Pop Out Video feature, also known as Picture-in-picture (PiP), allows you to display HTML5 videos in a floating window so that you can continue watching the video while scrolling down a page or changing tabs.

To use this feature, right click on the video and select Picture-in-picture from the context menu. For certain sites, like Youtube, this is not yet possible via their custom context menu. In that case, double right click to reveal the context menu with the Picture-in-picture option.

Pop out Video (picture in picture) window in Vivaldi browser