Recent changes to mover constraints made rail system unstable

Bug Reports Engine Bugs
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#1

I have a rail system that uses prismatic constraints, which guides a part along a series of complex tracks. Just the other day, the rail system was in perfect working order, no issues whatsoever. Today, the part that is supposed to follow the track was jittery and getting derailed.

I disabled this property to revert the behavior, and the rail system was working again as intended.

image

In this video, you can see the intended behavior of the track system. The ball follows the track smoothly with no hangups, jitters or issues. This is with this property DISABLED.

This video has the property ENABLED. The result is a high likelihood of jitteryness or hangup.

This jitterness does not happen 100% of the time when the property is enabled, but it is quite frequent. It does not seem to happen at all with the property disabled.

Expected behavior

I expect the rail system physics to act like they do in the first video, which is how it was working prior to today. (Could have been yesterday, I was not working.)

A private message is associated with this bug report

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#2

Hey! Apologies for the issues.

The new behavior with the property Enabled is likely the “correct” new behavior. This can break a small amount of places that were relying on the older root selection behavior, which is why we were making this change via a phased rollout (you can see details in this announcement Improved Mover Constraints: Enhancing Stability and Network Ownership)

Nevertheless, we have decided to rollback this change for now. Please note that this behavior change will still be re-enabled soon, and we highly recommend that you check out the announcement to see what needs to be done in order for your mechanisms to work properly with the new behavior :slight_smile:

Let us know if you have any more questions or run into other issues.

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