The Joseph O'Brien-trained gelding went to post a 5/2 shot for the two-miler and chased the leaders throughout.
Shortall sent his mount up to tackle front-runner Cobaltic two from home but he had to work to get the better of that rival going to the last.
Fellow market leader Fiveallin also tried to challenge at the final flight but Christopher Robin found more on the run-in to secure a three-parts-of-a-length win over Cobaltic.
Advanced Virgo stayed on for third, a neck back, and he was just three-parts in front of Fiveallin in a tight finish.
Christopher Robin had been well beaten at Navan on his previous run, when he lost a shoe, but had run well before that when third at Gowran.
Shortall said:- "On his Gowran run he was entitled to win this. He had an excuse the last day. Joseph found a good opportunity for him today and everything went right for him. He jumped and travelled well."
Early Doors provided trainer Joseph O'Brien with a second winner of the week and a much-needed tonic after the tragic loss of Sir Erec earlier in the day with victory in the closing Martin Pipe Handicap Hurdle.
The topweight, who finished third in this race last year, looked booked for second up the home straight besides the looming favourite Dallas Des Pictons, but ultimately found more under pressure for Jonjo O'Neill jnr.
"He's been running in Graded company all year and he came back to handicap company today.
"Jonjo gave him a great ride and I'm delighted for him and we're very happy. Horses jump for him and he's very patient. He didn't get the break but went to plan B and did it perfectly."
Band Of Outlaws gave trainer Joseph O'Brien his first Festival winner with a convincing two-length success as the 7-2 favourite under JJ Slevin in the Boodles Juvenile Handicap Hurdle.
The four-year-old started out towards the rear but gradually made his way through the pack before taking over after jumping the final hurdle to beat Gordon Elliott's Coko Beach by two lengths.