Following the Material design guidelines for swipe to refresh pattern, should the spinner come out from behind the header bar or is it fine if it appears from just below the header bar?
For Material Design, it should slide out from under the header bar. The reason is: the user may be sliding down from anywhere on the screen, so there is no spatial connection between the interaction and the icon.
For this interaction, the ENTIRE spatial connection comes from the sliding movement, which is why the movement of the refresh icon has to mimic the sliding movement of the finger (same velocity). If the icon just appears (or fades in), there is a psychological disconnect for the user between sliding the finger in one location and a refresh indicator appearing at another location. So the interaction feels less natural.
As a side note, I think this particular interaction for material design is dumb. Twitter's approach of sliding down the entire sheet is much more intuitive and frankly comports better with the physical dynamics of Material Design.