The Stacked Tabs feature in Vivaldi groups tabs into a single tab. This is Compact Stack Tabbing. The stacked primary tab can then be expanded to access the other tabs within. With version 3.6, Vivaldi now has a Two-Level Tab Stacks feature. With this feature, even when the tabs are stacked, they will be available below the primary stacked tab as a second-level and be visible at all times. Using Two-Level...