This had been bugging me for a long time. Gnome Shell Ubuntu decided that it would be nice if users couldn't get to applications while dialog boxes (like a save menu) were open. Here's a fix to undo that decision. src. This is functional on 17.04.
Detach dialog
dconf write /org/gnome/shell/overrides/attach-modal-dialogs false
Attach dialog
dconf write /org/gnome/shell/overrides/attach-modal-dialogs true
