I have a few observations about new layout, which seems to be great, but there's always some "but" :)
If we have an option to open sheets in browser tabs, including use of middle mouse button, why it is limited to the new left-hand side menu? It is not possible to do the same from the main window where all sheets are listed, which is a little bit pain.
Also the new menu could be lockable to avoid auto-hide after sheet open. If sheets can be pinned in there, so the whole menu could be pinned as well.
Both of these is very useful for admins, who has to jump through many sheets often, in my opinion.
Please let us know if this is on the road map ;]