Question: Up to what point can I call a discarded tile?

Answer: The “window of opportunity” can be defined as the period of time in which a player can call a discarded tile. The window of opportunity closes when any of the following five things occurs:

1. A player calls a discard and places the discard and/or tiles from their rack on the top of their rack.
2. A player discards a tile.
3. A player racks their pick from the wall. Please note, if a Player has picked a tile from the wall but has not racked it, a call for a discard takes priority. In this case, the player who picked from the wall must put the tile back in the exact position form where it came. Also, please note that tapping or clicking a tile on one’s rack is not considered racking.
4. A player announces a joker exchange.(Also see Id149)   
5. A player declares Mah Jongg on a discarded tile.

Closing the window of opportunity is each player’s responsibility and thus a reminder of why it is important to pick and rack your tile rather than hold it in your hand. 

Sources: Mah Jongg Made Easy 2024, pg. 17, #8 (a), (b), (c)