Can a player cast spells after the active player passes priority but before the end step ends?

Priority works as follows: whenever a player passes priority, the next player in turn order receives it (CR 117.3d). A step or phase only ends when all players pass priority in succession without anyone taking an action (CR 117.4). So passing priority is not the same as ending the step.

During the end step, the active player receives priority first. If they pass without acting, each opponent receives priority in turn order. Any player who holds priority at that moment may cast an instant-speed spell or activate an ability before the step can close (CR 500.2, 117.3).

This matters because the end step is a regular step with a beginning, a priority window, and an end — it does not simply close the moment the active player is done. Sorcery-speed spells cannot be cast here (CR 307.1), but instants, flash spells, and activated abilities are all legal as long as a player holds priority.

Concrete example: It is your end step. You pass priority with no action. Your opponent then activates Viscera Seer (a free activated ability) to scry 1 before the end step closes — perfectly legal, because they received priority after you passed it.

Only after every player has passed priority in succession with no actions taken does the game move to the cleanup step (CR 514), at which point the end step is truly over.