Riven Q ability
 

Riven's Q is allowing her to ignore stuns while she is useing it. As Renekton I used my W multiple times and it always got ignored while she was in the process of useing her Q ability.

Tried it with a friend in a custom game and while she is using her q ability you can't stun her with Renek's W ability.

The same thing happened to me for many times.

While Riven is jumping around with broken wings, she is immune to stuns.

It's not one of the cast animation vs channel stuff. She can just keep moving or attacking during the stun duration.

Lets say Riven is hit by a 2 seconds stun while casting broken wings.
She is able to finish her broken wings because it wasnt a channel'd spell, which takes less than 1 sec.
But in the remaining 1 sec of stun, she would not be stunned and would be able to keep walking or attacking.