The 24-year-old Direct Energie rider nearly came to a halt 5km from the finish in the heat as a tough day in the saddle caught up with him, but he still had a bit left in the tank.
Emanuel Buchman was the best-placed overall of all the riders in the group, the white jersey contender becoming virtual leader as a splinter group of 13 - and then 16 - took charge at the front. More unsafe contenders are closer still to the three-time Tour champion. Arguably the toughest of this Tour, it has seven notable climbs. "A monster stage" is how Froome described it, predicting the race standings will "get blown to pieces". That's just shy of the height of western Europe's highest peak, Mont Blanc, and about belly button-height on Everest. Three are "hors categorie", or defying classification. With an average 10 percent gradient, and even steeper than that in parts, it will push riders already exhausted by the previous six climbs to the very limit.
"We came into a very tight right-hand corner and I think Geraint hadn't got his line quite right", Froome said.
Complicating matters: Saturday's stage, also in the Jura mountains, was far from easy.
When Calmejane attacked with 6km of the climb - and 18km of the stage - remaining, only Dutchman Gesink of LottoNL-Jumbo had the legs to follow.
The question now is whether Sky will pay for the effort on Sunday and run out of juice on the 181.5-kilometer (112-mile) Stage 9 from Nantua to Chambery in the Alps, arguably the most grueling of this Tour's 21 stages.
"We had to ride a pretty fast pace all day", Froome said. "I can thank them for keeping the jersey".
GETTY Chris Froome went off-road when he misjudged a corner but managed to correct his error
Calmejane, who rides for the Direct-Energie team, shrugged off a bout of cramp towards the end of the stage to earn the biggest win of his career.
Calmejane's victory was the host nation's second in this year's Tour following Arnaud Demare's Stage 4 triumph in Vittel.
Froome leads the general classification by 12 seconds from Thomas, with Italian national champion Fabio Aru of Astana third, 14 seconds back. Thomas quickly rejoined the race, and Froome said his teammate was uninjured.
"One minute everything is going well, the next you're in the ditch with your team-mate over the barrier lying next to you".
Calmejane held off Dutch rider Robert Gesink, hot on his heels, on the final climb and rolling finish. "I've never been up there myself so I can't say what it would be like but if the Norwegian fans are anything to go by they would certainly give the Tour what it deserves in terms of a Grand Depart". "It would have been so sad to lose the stage like that".
Calmejane's team is one of 22 Pro-Continental outfits - effectively the second division of professional cycling.
Another Frenchman, Guillaume Martin (Wanty-Groupe Gobert), took third place.
"It's huge. It was everything I was dreaming of", said Calmejane, who won a stage in last year's Vuelta.