1) The maxUsedMemBytes for $_internalSetWindowField is expected to be non-zero, but it is currently 0 for explain.

2) $setWindowFields should clear memory on EOF. 

We tried to force the current memory tally to go to zero by calling memoryTracker.resetCurrent() in setWindowField's doDispose() method, but that triggered an underflow in memory when testing a $setWindowFields when followed by a $limit that forced the $setWindowFields to return early.