Considering how customizable Vivaldi already is, offering an optional way to repurpose double-click or middle-click for tab name URL/search input is a natural fit. Safari already does something similar, so it's not without precedent. It's the same with renaming!
Renaming a tab is fine, but what's the practical use of that compared to quick control and efficiency? The idea isn’t to truncate the main URL bar—it's to hide it entirely for people who don’t always need it. Editing and searching could happen directly in the tab title area. If there’s room to rename a tab, there’s room for this.
You can try it right now by renaming a tab; there's plenty of space for everyday searches. Alternatively, if you select all or press a button, the address bar can be lowered as a floating or typical panel.
This would be optional, like most things in Vivaldi.