At least for OSX the edit menu undo / redo commands don't seem to work in post editing mode. The key commands work normally. Maybe not the most needed thing but still.
OSX 10.14.5, Publii 0.34.1
I checked it a moment ago, and it seems that this option works as should. Could you attach a video with your issue?
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Actually what is seems to do is that the undo / redo acts rather erratically. When used from the osx menu bar I mean.
Attached files: undo redo.mov
I suppose that it can be some strange issue with TinyMCE and these buttons. In fact the best option would be hide these options from the top menu as the keyboard shortcuts works correctly, but unfortunately if we remove them, then the global shortcuts connected with them will also not work :/