sidebar tabs are so great. more horizontal space for the title, reduces tons of whitespace in 99% of sites, hierarchical collapsing in many (ya like tab groups? tree-style-tabs users have had them for a decade already), and it's easy to make them use little vertical space.

it's such a no-brainer, I'm surprised it's not a first-class feature in the main browsers (I'm aware that it is built in or default in a number of niche browsers).