I am very frustrated with the new format. Previously, every single sheet I had open was accessible with a single click, plus I was able to use the entire width of the screen for each sheet.
Now, in order to us the entire width of the screen for each sheet, it requires triple the amount of clicks to change sheets: One to open the side panel, one to select a different sheet, and one to close the panel. Good grief.
If I want to keep my one-click access to each sheet, then I give up space because I have to leave the side panel open.
I am curious if you tested these changes with actual users in your research, because the new design wastes both time and screen space, which is quite an accomplishment.