This is an option in tree style tab.Let's say u have a google search tab with lots of child tabs.
Then if u are finished searching on google, you can simply close it since you don't intend to search anymore.Then, you process the child tabs of that search.
However, the tree structure is lost once the parent google search tab is closed. All the child tabs will become root tabs.This makes the tree very messy. (see screenshot)
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