Think ATM/Cashpoint/Bancomat. Whether they're on-screen or real metal buttons, they're always there.
All appliances have an OK button. Call it an ON switch, call it what you will but there is a Big Obvious OK button (or that silly "Apply" button*) that begs "press me to do what i'm supposed to do". And it's always somewhere visible, usually right there on its face. Not in the other room hiding. Not in some other dimension waiting to be invoked by a ouija board.
Being how software that follows a windowed GUI model has no other side than its face (even when it's pretending to be 3D), the "press to continue" button always has to be visible.
Anything less, documented in the FM or not, is a secret handshake. And not spoiled.
As far as dragging a window without finding its title bar, that is an optional interaction. The "chrome" (i believe that's the term) is where all primary interaction with the window container happens. The contents of the window is where data interaction happens. I want to push pixels, i click and drag and dance inside a window; i want to move/resize said window to different coordinates, i drag its chrome around.
*"Apply" (imho) has always seemed a redundant way of being redundant. And repetitively and unnecessarily repetitive.