I like everything about the new viewer but one thing: when you give it the FEN for a position, the subsequent moves are forced to appear as starting with 1 again.
In other words, even though the FEN includes info about which move it currently is, it doesn't accept PGN for moves starting with the move number, only moves starting with 1.
See this post for an example. I included a FEN which shows that it is the 16th move, but was forced to number the moves as 1, 2, and 3 rather than 16, 17, and 18.
So, I propose that numbering either way (starting with move number or starting with 1) be made an option.