Since when has the timeline moved with a static playhead during playback?
I seem to be going mad this morning!
Normally during playback the timeline stays still whilst the playhead moves. Only when the playhead comes to the end of the visible timeline does the timeline jump to expose a new section.
However, my timeline is now moving whilst the playhead remains in the same position . . . well actually the playhead appears to be vibrating back and forth a few millimetres.
iMac 27″ 5K, macOS 11.5