You're both wrong! Mindbreak is when everyone gets visions, instead of just seers. It is interesting to note that the effect of constantly receiving visions is "irrevocable insanity".
On the other hand, when Dom checked his own future, he did not appear to enter mindbreak, so much as turned into some sort of furnace, I guess.

Basically seers are incredibly dangerous, and the setting never really reconciles this with their cavalier treatment. Sometimes they just throw seers down flights of stairs.