Comment 15 for bug 558327

More likely, in my view: The human theme has a very visible prelight for the close button. The Ambience theme does not; there is really no way to tell whether you are hovering over (or even clicking) the button unless you're a pixel counter :b

Thus, one could have the pointer positioned at the top right and not realize that, upon clicking, they will hit the close button. So, someone tries to drag the window from the top left, but nothing happens (pointer doesn't change). He releases the mouse button and the window is unexpectedly closed.

Was that patch actually being adopted into official upstream Metacity? If not, it's a very significant divergence. I recommend holding off on it, maybe trying to fix the problem from Ambience first.
(Having said that, I for one am content being able to drag the window from the corner).