Writing tags back to music files can be very slow, particularly to a
network filesystem, and blocks the UI; quodlibet sensibly puts up a
progress bar.  However, the progress bar is implemented as a dialog box
that isn't attached to the application window, can't be moved, doesn't
go away when quodlibet is minimized, and insists on being on top of all
other windows.  This means it's difficult to do something else with the
computer while waiting for it to finish.