Split-view undo history preview
What about a split view for the undo history, e.g. showing current state in one half and selected history state in the other, or having a control-click (say) to preview?
A possible difficulty is that right now you click to revert to an undo state so either the preview would always compare with the most recent entry in the list, or you'd need another way to click to undo.
But it would give a way to compare the results of several filters, for example.