...
Each running virtual thread is mounted on a carrier thread. There are a limited number of carrier threads (usually defaulting to the number of available cores). When a virtual thread hits a debugger breakpoint, it continues to "occupy" pins its carrier thread, preventing any other virtual thread from running on it. If you have an application with a large number of virtual threads, and you setup a breakpoint that many virtual threads will hit, you can wind up in a situation where every carrier thread is running a virtual thread that is at a breakpoint. No virtual threads will make progress when this happens. Those that are mounted are all at breakpoints, and those that unmounted have no carrier thread to run on. If you resume one of the virtual threads, when it yields it will allow unmounted virtual thread to run, which likely itself will then hit this same breakpoint, leaving back to having all virtual threads being stuck.
...