Ajit is playing against Niko at a Modern PPTQ. Ajit casts Spreading Seas targeting Niko’s Glacial Fortress and says “Trigger, draw a card”. Niko stops Ajit from drawing, floats one white mana, then allows Spreading Seas to resolve. Ajit thinks for a minute, then says “Go to Combat?” Niko taps one Mountain and casts Lightning Helix targeting Ajit’s Snapcaster Mage. The players put their cards into their graveyards, then Niko calls for a judge, and says “Ajit missed his Spreading Seas trigger.”
What do you do?
The key piece of information that many missed was the fact that Niko must be acting in Ajit’s Main Phase, since the floating white mana was used to cast Lightning Helix.
Since Niko is acting in Ajit’s main phase, Ajit using the combat shortcut is no longer relevant – he effectively only passed priority once since Niko responded immediately. Niko could have been acting with the Spreading Seas trigger still on the stack, so we treat it as such. No game actions have been taken that indicate we are past the point where the trigger should have resolved.
There is no infraction, the Spreading Seas trigger is still on the stack.