F3 / F4 are not doing anything (as seen in WL6). In “Customize commands” I can see CTRL+Z / CTRL+SHIFT+Z assigned (they do work) and when I try to change them I get message that F3 / F4 is “Already used by Undo / Redo” 
Also when I try “Search by keyboard shortcut” there (F3/F4) I get no results…
What am I doing wrong?
When you try this to assign F3/F4, you indeed get this weird message. But ignore and continue, and F3/F4 can be used as you expect.
I see - I tried and it worked (e.g. assigning to F3/F4).
Is there any way how to keep CTRL+Z/CTRL+SHIFT+Z assigned too? In other words how to achieve the same behaviour as in WL6?
This is how it is intended to work b default, but apparently it does not. I have to check why.