Is it possible for a container citrus that has healthy roots, but has dropped almost all of its leaves (due to stress in a store), to start growing new leaves on a windowsill with 8 hours of daylight (almost no sunshine), at 17-19C (62.6-66.2F)?

Oh, and one more thing: soil temp - 18C (64.4F) right now - at least.

I'd really like to know

Evaldas, I'm no expert but from what I have seen from my seedlings I think it is possible. I had seedlings putting out new growth in this cold weather at 10C, then my heating failed and the new growth was damaged by freezing temperatures. Good luck with your tree, I hope it grows well.

Perfectly normal, mine is growing now. It's 5-10C air temp, 15-20C soil temp and little sunshine. Been growing for 3 weeks