The second of the two big causes of PvP desync is the fact that occasionally, one player starts the game while the other player is still searching for their opponent.
The fix for this one is easy.
Simply add a [Ready] Button to this screen, before the timer starts:
The game cannot start until both players have hit the ready button.
This confirms that both players are in the game lobby before the game starts, meaning no chance that one player will enter the game before the other one.
Simple no?
