Here's a contrarian opinion: there's no such thing as seeing a fully rounded earth!
Standing at the top of a high building, you look out from the center of a circular (fully round?) disc (with a hump from the spherical earth). The edge of that disc is the horizon, the farthest point you can see in any direction.
Since you are above that rim, you are looking down very slightly. Navigators using a sextant have to account for this "dip of the horizon" when making sextant observations of the sun or stars. The height of the navigation bridge above the water is a correcting factor in their calculations.
As your altitude increases, two things happen. The distance to the horizon increases, but more slowly as you rise. Secondly, you are looking farther and farther down to see this (fully rounded?) disc, as you rise farther and farther above it.
B as you rise you are always looking at a round disc, from a slowly changing perspective...