Thanks for reaching out and for your interest in CryptPad.
Fast Mode is real time collaboration and hence cannot support undo functionality. If two persons do it at the same time, what would happen? How would you handle the conflict between the two changes?
That's why we propose instead the strict mode as explained in our user guide. Which then allows to use undo/redo functionalities.
Regarding your question about the menu, you are likely getting confused by the fact there are two File menu buttons. See the screenshot below.
We know it's quite an issue, degrading the user experience but haven't been able to find a viable workaround yet.
Hope this helps!