Continuing the discussion from Undo Doesn't update in Preview Panel Version 2.3b (850):
The preview panel does not update at all, not only when undoing. Added a new shape to a glyph and it didn’t appear on the preview panel. The only work-around was to resize the panel, then the window was updated (see recording: http://g.recordit.co/6QF45AwZfu.gif). Not the most efficient method indeed. Remarkably, it does update when side bearings are changed!