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": http://cgit.freedesktop.org/libreoffice/core/commit/?id=e681749ffa5a18c8a6ff874787d4ab8411a46125 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: http://wiki.documentfoundation.org/Testing_Daily_Builds Affected users are encouraged to test the fix and report feedback.