Thanks! That solved it, and it's not even repeatable it seems, at least by basic means. Though I was lucky not all of them was located in a negative screen position, then I could'nt have reached the titlebar and they would've been stuck like that.
Update: It is repeatable, you have to be careful at what positions you put each dialog, as each dialog have their own relative screen position offset, that is kept when they're maximized, but the maximization toggle is a common setting. But that makes a situation where all dialogs end up on negative positions seem to be impossible from basic user perspective.