Triggered abilities are a constant source of uncertainty for players. They can trigger pretty much anytime, but that doesn’t mean they can go on the stack at any moment. After they trigger, triggered abilities just kind of hang out in a limbo-esque waiting room until their number is called the next time a player would get priority. That means anything that’s currently resolving has to completely finish first, then state-based actions are checked/performed, and only after all that can the trigger actually get put on the stack.
So let’s say you cast Demonic Tutor. You search, you put a card into your hand, and you shuffle. Widespread Panic triggers. After the shuffling is done, the trigger from Widespread Panic goes on the stack. This means if you searched for a non-instant/non-flash card, you won’t get a chance to cast it before you resolve that trigger. If it’s the only card in your hand, you’ll have to put it on top and draw it again later… but at least you know it will stay on top!
What if you have no cards in hand as the trigger resolves? Maybe you searched for a Lightning Bolt, and cast it in response to the trigger. Maybe you cast Mystical Tutor with no other cards in hand. Maybe you had no cards in hand and you cracked a fetchland like Misty Rainforest. Well, you simply can’t perform the impossible action, so nothing happens as the trigger resolves. Lucky you!
But what if you cast Green Sun’s Zenith? This is a peculiar card because it tells you to shuffle twice; once after the search, and once as the owner shuffles it into his or her library. Anyone who has cast this spell knows that the duplicate shuffle is mostly silly, and everyone shortcuts that into a single action, shuffling the card into the library. So why is it written that way? Well, there’s the unusual situation where a player casts and/or gains control of Green Sun’s Zenith but doesn’t own it! It would be strange indeed if I used Praetor’s Grasp to cast your Green Sun’s Zenith, then searched for a creature, but only the owner had to shuffle! The side effect of this is that even if you shortcut to combine the two physical shuffles into one, the game knows you shuffled twice, and Widespread Panic triggers twice. You’ll then have to face the sad reality of resolving only the last part of Brainstorm (by putting 2 cards back on top of your library).
One last note: Widespread Panic specifically says “…a spell or ability causes its controller to shuffle his or her library…” so you can’t get extra value out of something like Chaos Warp, since your opponent is only shuffling because of a spell YOU control. Also, going back to the first paragraph of this post, even though the shuffle is at the beginning of Chaos Warp’s resolution, the trigger from Widespread Panic doesn’t do anything until after the top card is revealed and possibly put onto the battlefield (so you can’t “cheat the system” by using this combo to put big stuff on top of your library for value!).
Today’s Rules Tip written by Josh Stansfield