Dan Linden Thank you for bringing this report to my attention Dan :)

I used Firestorm and latest Second Life viewer both. The only difference is Second Life viewer does not have all the features that Firestorm viewer has. So the same issue cannot be recreated in different scenarios which would enables us to look at some things in different angles and understand things better which in return helps fix issues sometimes. I have now added a possible fix for this to try (commented later here).

I assumed that if my assumptions were right then it should do the same on closing avatar pose mode as well and sure enough it does the same on closing avatar pose mode. And in both reproduceable instances (exit from pose mode and exit from sit) the running in one place animation does not stop only on triggering of movement controls but also stops if you click in the viewer viewport. Clicking on viewer UI does not stop it as it is only on client side. But needs to be a left mouse button click within viewport or any interaction like movement controls within viewport (part of the world thats drawn for user).

notes;

1.) Collision (avatar collider physics state) handling between sit/posemode and stand modes.

2.) How that affects the animation transitioning from frozen physics back to normal.

3.) https://wiki.secondlife.com/wiki/Collision says "A collision with a hovering avatar does not trigger collisions, unless the avatar turns or moves". Maybe this should be checked to see if the avatar is clicking on in world at the same time collision happens if that collison is detected as well.

4.) Is the avatar collider mesh in a hover state when sitting or in pose mode?

5.) How does the handling of avatar physics managed from pose mode/sit to stand? Would this transition into thinking the avatar is running/sliding downhill?

Dan Linden Fun test to do that might help understand this part of physics interference. Create a physics ball and let it fall from sky. While it is falling click outside the Firestorm viewer. You will not see any interference to the physics as it continues to fall down smoothly. Now make that same physics balls sit-able on click (right click balls - edit - General tab - Click to "sit on object") and edit it and move it back up to sky. Let it fall back down and click it to sit on it while it is falling. Then quickly click outside viewer while it is still falling. The ball will see interference to its smooth falling. Meaning for a split second it will freeze mid air before it starts falling again. Whats is the difference here? Why did it freeze mid air for that split second?

Dan Linden Further more atm on ground flat terrain this seems to be more the case than on firestorm sandbox prim floor at height 2702 where the Avatar does stabilize itself and come to a stop and stand animation after a one second run. It occurred to me to test on terrain vs on prim again only now. I now realise this issue was originally reported back in 2015 and they say its the case on flat prim and terrain both. And as my memory serves me when I tested this earlier today it was the case on flat prim too. Now it seems the bug is debugging itself when avatar is on a flat prim so maybe you guys are already fixing (silent update?) this issue :)

Dan Linden Latest observation & possible fix to try.

On flat terrain.

Go into pose stand mode. Enable (tick ON) the lock tick button. Close the Pose Stand. You will observe avatar running in one place.

Now take one step back. Do not rotate/turn. Only 1 step backwards. Go into pose stand mode again. Disable (tick OFF) the lock tick button. Close the Pose Stand. If the avatar has not stood without running do nothing but again just 1 step back (no turning/rotating please) and go back into Pose Stand mode. Keep the lock tick button Disabled (tick OFF). Close the pose stand. You should see avatar not running on flat terrain.

If you move forward/sideways/turn the physics always add force in that direction which could trigger run animation. Taking 1 step back applies a force backwards to cancel out any force still running in physics. Just because you get avatar on pose stand mode does not seem to be cancelling out the physics force to zero as it seems. So as they exit Pose stand mode the avatar shows a running animation due to the accumulated/left over force in physics recognising as avatar running (this depends on how the backend AI code is done obviously).

Possible fix to try : Zero out the force in avatar physics on exiting Pose Stand mode and sit mode.