A modal dialog is a window which, once opened, will not allow the user any interaction with the window below until the dialog is closed.
In GNOME 3 the default modal dialog behavior is to attach the window so both the back and front window will be firmly attached together and dragging one of them will move even the other window, like if they became a single window.
This behavior can be changed to restore the old way the modal dialogs were treated in the previous GNOME releases. To detach the modal dialogs from the main window you will just need to execute the following command (in a single line) inside a terminal window:
gconftool-2 --set /desktop/gnome/shell/windows/attach_modal_dialogs false --type bool
To attach the modal dialogs to the parent window you could execute:
gconftool-2 --set /desktop/gnome/shell/windows/attach_modal_dialogs true --type bool
After issuing one of the previous commands you’ll need to restart the GNOME Shell by logout the current session.
The GNOME Shell 
Comments on: "The modal windows behavior" (6)
Thanks. Very easy to find on the web. I hate the attached dialog window setup. Often I need to move the window to see some file entry behind it.
It’s nice not to have to interact via a socal network site. I don’t use those.
This won’t work on my Ubuntu/Gnome 3 desktop.After reboot, modal windows still are atacched on top, althought “gconftool-2 –get /desktop/gnome/shell/windows/attach_modal_dialogs” returns “false”.
This no longer seems to work…
The article is quite old and the last reply was a year ago but I think it’s good to mention here if anyone come to this blog when looking on how to detach modal dialog.
The setting is now available under /org/gnome/shell/overrides/attach-modal-dialogs
Therfore, to detach it:
gconftool-2 –set /org/gnome/shell/overrides/attach-modal-dialogs false –type bool
I was looking to detach the modal dialogs too, and through my search this page came up; even though it’s apparently a year old since last comment, I will post what worked for me anyway.
sudo apt-get install dconf-editor
Open ‘dconf-editor’ (no quotes) and edit: org.gnome.shell.overrides.attach_modal_dialogs
In other words, navigate with dconf-editor, org > gnome > shell > overrides (the key is in the right hand pane and labled “attach_modal_dialogs” and uncheck the box. Behavior was immediate.
Bobs post on 4 October works for me (Debian wheezy). I need detach for spell checker pop ups … often the spelling “error” in the document is covered by the spell window.