When using the two-level tabs bar (locked to always show the two levels), opening a new tab in a stack consisting of a single tab does not work correctly. Regardless of the method of opening a tab - the "+" button, context menu, Shift/Ctrl+left mouse button, mouse shortcut - the tab is opened outside the current stack, as a new top-level tab. The same behaviour applies to duplicating the tab. This happens regardless of the new tab position setting.

Everything works correctly when there are two or more tabs in the stack.
The same issue has been reported for macOS.

Along with other tab and stacks issues (broken mouse gesture, new tabs opening to the left of existing ones - this one might have been fixed in today's update) introduced since Vivaldi 7.9 it gets really frustrating. All the errors have been reported on the forum immediately, but they are left intact through multiple updates, including a major 8.0 release. It's great that you keep introducing new features, but how can anybody enjoy them if the core ones (opening a new tab!) get broken along the way and do not receive fixes?

Wersja: 8.0.4033.54 (Wydanie oficjalne) (64-bit)
Wersja Chromium: 148.0.7778.282
Kanał: Wydanie oficjalne
Platforma / OS: Windows 11 Version 25H2 (Build 26200.8655)
Ścieżka pliku wykonywalnego: C:\Users\adams\AppData\Local\Vivaldi\Application\vivaldi.exe
Wiersz poleceń: "C:\Users\adams\AppData\Local\Vivaldi\Application\vivaldi.exe" --origin-trial-disabled-features=CanvasTextNg|WebAssemblyCustomDescriptors --restart --flag-switches-begin --flag-switches-end