This toilet has been dry and out of use for several months and is about
two years old. After reconnecting it with a new seal and the cold water
line tight with no leaks, it leaks like a gushing river whenever flushed.

It's fine sitting with a tank full of water - no leaks at all. But the
minute the toilet is flushed, what a mess of water on the floor.

Which seal/gasket do I need to replace?