setUndoManager prevents access to existing history

[johngodley]

If you are editing some content and have built up an undo history, and then setUndoManager is called, it prevents access to the existing history.

This may occur, for example, on P2 where you can be creating a new post (collab is disabled) and a draft is saved. At this point, collab mode is enabled (the conditions for it are now true) and existing undo history is lost. Given the right set of circumstances this could result in data being lost.

[johngodley]

You can also get the same effect if you start typing in an editor before setUndoManager is called. In my testing there's sometimes a 10 second gap from when the editor is editable and the undo manager kicks in.