I mean 100,000 feet+, like this. (and I think you knew this also, Tom).
I don't see the earth curving. I just see the camera looking down upon the distinct circular area of the sun's light.
OK, lets forget about the causes of curves and look at some curvy maths. From Wikipedia the article on
Horizon has the following formula for estimating the apparent curvature of the horizon:
"A curvature of 1 appears as a circle of an angular radius of 45° corresponding to an altitude of approximately 2640 km above the Earth's surface. At an altitude of 10 km (33,000 ft, the typical cruising altitude of an airliner) the mathematical curvature of the horizon is about 0.056, the same curvature of the rim of circle with a radius of 10 metres that is viewed from 56 centimetres."So, from a height of 10 meters above the earth's surface (assuming R is 6,378,000m) the visual curvature (κ) will be
κ=0.0018.
Compare this to a balloon at a height of 3000 meters:
κ=0.031So... What does this all mean? If the curvature of the earth is right at the limit of perception from an airliner (κ=.056), then it is very unlikely to be visible from sea-level (0.0018 -> κ is 30 times smaller for sea-level).
Therefore the window test is not reliable experiment to determine if the earth is flat or round.