Since there is no quick access toolbar like in ms office there is no undo and redo button in the tabbed notebookbar design yet. Would be a importaint thing to add. It exists in the not-tabbed version, but i cannot find it in the tabbed version of the design.
Yes it is a known issue that we would need to address in future iterations.
Szymon Kłos committed a patch related to this issue.
It has been pushed to "master":
tdf#105040 Notebookbar: added shortcuts toolbox
It will be available in 5.4.0.
The patch should be included in the daily builds available at
http://dev-builds.libreoffice.org/daily/ in the next 24-48 hours. More
information about daily builds can be found at:
Affected users are encouraged to test the fix and report feedback.