setUndoManager prevents access to existing history

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.

Start by tracing the setUndoManager entry point and how it is called when collaboration mode changes. Reproduce the transition from editing with an existing undo history to enabling collaboration after a draft save. Done means the prior undo history remains accessible through that transition, with the data-loss scenario no longer occurring.

Written by the indexing model from the issue text.