Could I suggest it should be relatively straightforward to implement an optional layer in the Renderer Options of the GCode Viewer that contained the logical OR of all the pixels painted on the layers that have already been printed. These could optionally be painted in a light colour to generate a more realistic image of what has been printed to date even if printing has progressed above the height of some features (so it would be an alternative to 'Also show previous layer'). I think this would make the printing of certain features much clearer, for example vertical holes, and also understanding orientation when a print 'thins out' extensively on the upper layers.
PrettyGCode plugin....
Thanks for the tip. Yes that is nice and will be particularly useful for 'pre-flight checks'. However I still think my suggestion is worthwhile as well, offering a simpler top-down 2-D view. It would also be part of the 'out of the box' experience discoverable by a wider set of users more into printing rather than tinkering with the tools!