To clarify - I’ve been using UE4 since it released years ago. 4.22 versions of the scenes don’t have this issue - sends it to swarm and I can still work within the editor. 4.23 sends to swarm, but the editor is frozen with a progress-bar that typically finishes being stuck on screen (hopefully only) until Swarm / Lightmass finishes. It looks like I avoid the problem by choosing to just build the lights only (vs the general Build button I typically use).