The code is weird, I admit, but if I try this:

np.ma.extras.flatten_inplace([[1, 4], [2, 5], [3, 6]])

The result is not correct after this change, so I think the while loop is necessary here, because the sequence length can change.

Your right. My code is incorrect.

It's interesting that flatten_inplace was always used by the masked version of this function (61f9f6d), but never the unmasked version.

At some point we should revisit #8511, which would remove the final caller.