Tab Stack ordering
I really like Vivaldi's ability to stack tabs. As someone who often has multiple pages up for different things I'm doing, the ability to group them is really nice.
What's not really nice is the inability to order the tabs within the stack the way I want. I have one page that is the most important out of the stacked tabs - but there is literally no way for me to consistently make it the front of the tab stack. When I move the tab stack from one spot to another spot in the tab bar, the tabs within the stack reorder themselves. Even just turning off tab stack reordering when moving the stack, and adding the ability to right-click the tab stack and "move active tab to front of stack" would be great - it might be unwieldy to reorder multiple tabs this way, but it would be less irritating than having to open up a full menu or something to reorder tabs.