Regression in Buoyancy Example, neutral vehicle slowly sinks

[arjo129]

Environment

• OS Version: Ubuntu 20.04
• Source or binary build?
  Source 97f2e9c

Description

• Expected behavior: The center vehicle in the neutral Buoyancy example in the buoyancy.sdf example is supposed to remain stationary.
• Actual behavior: Instead it is sinking.

Steps to reproduce

1. ign gazebo buoyancy.sdf

More details

It seems like the cause is the way the buoyancy plugin is structured. In #1064 a change was made to prevent newly spawned objects from developing a spin. To do this, we skip one iteration of the update if the pose is not ready. Unfortunately this leads to problems as it gives an initial velocity to objects. Without damping this leads to the slow sinking we see above.

[chapulina]

Let's keep it open until graded buoyancy is fixed too