After loading a long-running aggregation pipeline with '$merge' in the end into the builder, its last stages time out, the stages are marked (correctly) with their red timeout frames, then blocks the GUI with CPU load of up to 200%.

Removing the '$merge' (thanks for saving the pipelines in files now!!!) from the end makes it behave normally again: times out, shows red frames, but GUI usable.

Marked as 'critical' because such a pipeline is unusable and more or less 'lost' until fixed manually at the moment.