How can I stop the game from pausing when the window doesn't have focus?
Solution 1:
Pokemon Uranium is made in RPG Maker XP. When tabbed out, all scripts stop running, but all audio continues playing. This can be seen documented in this forum post. There is no way to fix this, as it is an engine limitation. This limitation was removed in RPG Maker VX, but Pokemon Uranium runs in XP, and it seems this limitation is unlikely to ever be removed.
Further Source