When windows are ungrouped you can drag an item (i.e. file) to an item in the window list and it (the application you hover) is pushed to top so you can (for example) drag a file into a vlc playlist.

But when windows are grouped and you hover over a grouped application with a dragged file, the list of grouped applications does not pop up so you can not choose a window to drop the file into and the applications stay in the background.

This is really bad behaviour since the windows would not be grouped if you had enough space on your display to have all applications side by side for easy drag and drop.

Ubuntu 10.04
GNOME 2.30.3
window-list-applet 2.30.0