Traditional game: prevent going to another player's spawn area off-screen?

[Akuli]

If you move sideways a lot before your block is fully visible, you may end up blocking another player from spawning, because their freshly spawned block would overlap your block. This is not very intuitive when the other player suddenly gets to wait 30sec for doing nothing.

A good solution would be to prevent you from going to the other player's area off-screen, except that adding and removing players then becomes more difficult. Your area can shrink and expand (currently 10 units for single player and 7 for multiplayer), so what should happen if your block ends up partially on someone else's not-yet-visible area when they join the game?