Deadlock in Lin Bytes test with Thread on Cygwin

[jmid]

Opening this to track it separately from #520, as it will allow to distinguish occurrences of one from the other.
Also, in comparison to #520, this on a Threads test - and not involving shrinking.

On the merge to main of #519 for the Cygwin 5.3 workflow we just saw excessive time spent in Lin Bytes test with Thread
https://github.com/ocaml-multicore/multicoretests/actions/runs/12808801661/job/35712309410

random seed: 366364260
generated error fail pass / total     time test name

[ ]    0    0    0    0 / 2500     0.0s Lin Bytes test with Domain
[ ]    0    0    0    0 / 2500     0.0s Lin Bytes test with Domain (generating)
[ ]   37    0    0   37 / 2500    60.2s Lin Bytes test with Domain (shrinking:   15.0003)
[ ]   37    0    0   37 / 2500   122.0s Lin Bytes test with Domain (shrinking:   18.0006)
[ ]   37    0    0   37 / 2500   182.5s Lin Bytes test with Domain (shrinking:   20.0008)
[ ]   37    0    0   37 / 2500   245.1s Lin Bytes test with Domain (shrinking:   23.0005)
[ ]   37    0    0   37 / 2500   308.1s Lin Bytes test with Domain (shrinking:   25.0002)
[ ]   37    0    0   37 / 2500   371.9s Lin Bytes test with Domain (shrinking:   28.0010)
[ ]   37    0    0   37 / 2500   433.4s Lin Bytes test with Domain (shrinking:   30.0013)
[ ]   37    0    0   37 / 2500   493.5s Lin Bytes test with Domain (shrinking:   32.0012)
[ ]   37    0    0   37 / 2500   556.0s Lin Bytes test with Domain (shrinking:   34.0002)
[ ]   37    0    0   37 / 2500   617.4s Lin Bytes test with Domain (shrinking:   35.0012)
[ ]   37    0    0   37 / 2500   677.9s Lin Bytes test with Domain (shrinking:   36.0015)
[ ]   37    0    0   37 / 2500   741.5s Lin Bytes test with Domain (shrinking:   37.0013)
[ ]   37    0    0   37 / 2500   806.4s Lin Bytes test with Domain (shrinking:   39.0004)
[ ]   37    0    0   37 / 2500   866.8s Lin Bytes test with Domain (shrinking:   40)
[ ]   37    0    0   37 / 2500   927.3s Lin Bytes test with Domain (shrinking:   41.0003)
[✓]   38    0    1   37 / 2500   984.5s Lin Bytes test with Domain

[ ]    0    0    0    0 /  250     0.0s Lin Bytes test with Thread
[ ]    3    0    0    3 /  250  1481.8s Lin Bytes test with Thread
[ ]    4    0    0    4 /  250  8943.0s Lin Bytes test with Thread (collecting)
Error: The operation was canceled.

That's 149min ~ 2h 29min to test 4 random inputs on the Thread variant? 🤔