What can't poppy's w stop?

It can't stop a roundhouse kick delivered by Chuck Norris, nothing can. Pray he never becomes a champion.

When she opens a can of Pringles the fun doesn't stop.

It stops anything that is a dash, but isn't cc-immune like Vi and Malphite's ults. Anything that's actually a blink or provides intangibility is unaffected. Azir's wall functions the same way when determining whether or not something can pass through it.

Google it.

Steadfast Presence will stop only one dash per enemy champion per cast. Steadfast Presence does not stop: Diana's Lunar Rush (probable bug) Fizz's Playful / Trickster Hecarim's Onslaught of Shadows Jarvan IV's Cataclysm Kalista's Fate's Call Malphite's Unstoppable Force Maokai's Twisted Advance Nocturne's Paranoia Rek'Sai's Void Rush Sion's Unstoppable Onslaught Udyr's Bear Stance (possible bug) Vi's Assault and Battery

google Arkadian Did you know poppy. There you go. It also doesnt stop blinks like ezreal, shaco, or flash.