I'm more and more convinced that Deino is correct and we are looking at the FC-31v2 rather than the J-20 here. They may be difficult to tell apart from the section shown, but I've found one feature that supports the FC-31 theory:
Furthermore, if we assume that my own hypothesis of a canopy jettison (either to determine its trajectory as a hazard to people in an emergency on the ground, or in an ejection test) is true, that doesn't fit a J-20. It has a detonation chord to shatter the canopy - it probably doesn't get jettisoned at all! Same with the AV-8B or T-45A, for example - they shatter the canopy even in a rescue on the ground.
Furthermore, if we assume that my own hypothesis of a canopy jettison (either to determine its trajectory as a hazard to people in an emergency on the ground, or in an ejection test) is true, that doesn't fit a J-20. It has a detonation chord to shatter the canopy - it probably doesn't get jettisoned at all! Same with the AV-8B or T-45A, for example - they shatter the canopy even in a rescue on the ground.