Scrubbing the timeline in Sequencer can hang the editor when operating on constrained actors with complex control rigs

The issue is caused by UAnimDetailsProxyManager::ForceUpdateProxyValue triggering evaluation of constraints that are attached to control rigs. When scrubbing, requests to ForceUpdateProxyValue are repeatedly queued during the scrub action and then executed when the scrub stops. This means we then call ForceUpdateProxyValue many times, evaluating whatever the constraint is attached to many times, which can cause frame rate issues or hangs when it’s a complex control rig.

• Open the attached project
• Open the level sequence ANM_SH044x00-98
• Navigate the camera so you can view the BP_C_Piggie_PRD actor
• Scrub back and forward quickly in the sequencer timeline, keeping the playhead between 0 and 40 frames
  • This is when the constraint is active
• Stop scrubbing
  • Note how the framerate of the editor significantly decreases for quite a while or can even hang