I'm unable to change shortcut for built-in search engines. In Chrome I always used 'y' for youtube and in Opera it is reserved for Yandex.

I already did that. My patch is pretty dirty, I hope author will make proper changes to support Opera browser.

Be sure to remove the whole title bar when user hides tabs to save space. 'Opera' button should probably transform to sandwich button in toolbar (like in Chromium) and close-minimize buttons should also move to toolbar.

If someone wants more sophisticated tree tab manager there is Sidewise plugin for Chrome. I patched it to work with Opera's sidebar. May be glitchy but it works for me. You can download it from my Dropbox. Diff file is included in archive.

And the option to remove top tab bar (which is a bit confusing when we have tabs in a sidebar)

Now we just have to wait when Tree Style Tab will be ported to Opera