Currently, it only waits for balancer round number to go up:

https://github.com/mongodb/mongo/blob/ffc6e38bc90750cc639317fc713e7bc4e0bf596e/jstests/sharding/clear_jumbo.js#L89

However, this is problematic because the balancer round also gets incremented every time the balancer skips (and does nothing). Instead, the condition should be "balancer in mode: full for more than 1 round".