Another undocumented feature I didn't know about but just came across. If you set the line attribute and closed shape attributes differently, then if you start drawing a line, stop, then with a second click, carry on to complete the shape (such that the attributes change from line to closed shape), then you can press undo once to revert the closed shape line attribute to the open shape line attribute. Not as useful as the recent nested-group select discovery, but still useful. If you draw and close the shape in one mouse click, then it doesn't work because there isn't anything for the line to change back to.