Yasuo's Q does no damage to 
Poppy if she is charging at him with her E. I first noticed this during a 5v5 on Summoner's Rift and went to a 1v1 in the Howling Abyss with a friend (DOM450) to confirm that I didn't just imagine it. Riot, please fix this bug. andiguessyasuo'swindwallshouldblockpoppy'schargewhatever.
hi im DOM450 his freind to tell you this is not a lie lol.
I'll just assume this is an intended interaction because THAT'S just how invulnerable Poppy is.
To be fair, Yasuo has many shaky interactions, like Kayles ranged autos going through windwall, so this is an extremely fringe one that probably won't matter 99% of the time.
At Poppy's charge speed, I don't know if you could time it right anyways. My bet is that Poppy charges into Yasuo's Q area after the actual damage proc in the area, and you're judging this by the animation lingering. The lingering animation doesn't represent a damage field duration necessarily.
I could be wrong, though. Haven't tested this by any means.
