Please describe the problem:
With a multihead X server running in xinerama mode, when a new folder is opened
for the first time it is centered based on the entire width of the desktop. 
This means that the folder window is split across both screens

Steps to reproduce:
1. Right click on the desktop backgroun, select "Create folder"
2. Give the folder any name you want, eg "Test"
3. Double click on the new "Test" folder to open the window


Actual results:
The window is centered on the total width of the desktop.

Expected results:
The window should be opened centered on the screen that has the mouse pointer.

Does this happen every time?
Yes

Other information:
I've opened this under Nautilus, but on reflection it may be a window manager
bug (Metacity, 2.8.6)?