@Cody Murphy good question, and honestly this is one of the more important edge cases for anyone running the kit in a long-term outdoor deployment.
The supercapacitor design inherently handles this better than a battery-based system would — supercaps can source current almost instantaneously with no chemical reaction delay, so the handoff on a hard input drop is going to be much cleaner than you'd get from a LiPo with a protection circuit in the way. That said, I'd want to flag this specific question to Moamen directly rather than give you a confident answer on the exact dropout window, because the behavior really depends on how the power management circuitry is implemented — whether there's a diode-OR arrangement, an ideal diode controller, or active switching between the input rail and the supercap bank. Each of those has a different transition characteristic, and a few milliseconds is exactly the margin where it matters which one is in use.
For your use case — continuous GNSS logging with active writes — I'd honestly suggest also handling this on the firmware side regardless of how clean the handoff is. A brief write-safe flag before committing to flash, or using a circular buffer that only finalizes records after confirming stable power, will protect you from the occasional edge case that hardware alone can't guarantee. The Dev Hub would be a good place to post this and get Moamen's eyes on it — he's pretty hands-on with technical questions like this and can give you the actual circuit behavior straight from the design.
