> Stop-the-world is only brief, to scan the register file of each thread and any other capabilities held in the kernel (asynchronous I/O, signal handlers, etc). Once done the entire memory sweep can be done concurrent with execution.

Is there documentation of the scheme that allows this? I did some quick Googling and found CHERIvoke (which is not concurrent at all) and Cornucopia (which requires scanning some user pages while the world is stopped). Are you referring to something newer?