I like the new sidebar, it would save me a browser extension if it would be tweaked just a little.
If you look at the extension "tree style tabs" they do it a little better or more convenient at least.
When I open a new sub-Tab from an existing Tab the new one will be below and slightly to the right
So you can see that it is under the "master-tab" and you can minimize all sub-Tabs by clicking
a little arrow on the master-Tab.