Vivaldi Technologies released Vivaldi 3.6 Stable to the public on January 28, 2021. The new stable version of the Chromium-based browser improves the tab stacking functionality of Vivaldi significantly. We reviewed the feature when it launched in snapshot builds back in December 2020. You can check out the review here.
Tab Stacks is a feature of the browser that allows users to stack tabs on top of each other. All it takes for that is to drag and drop tabs on top of each other. Besides using it to group sites together, e.g. multiple pages from a single website or pages from different sites that discuss the same topic, it is used to free up space on the tab bar as a single tab is used for a number of open sites when the feature is being used.