What happens if I hit File->Save, then while the file picker helper process is spawning I get an urgent instant message and tab away, and when I alt-tab back focus returns to GNOME and I close the tab because I forgot I didn't finish saving? Does the file picker go away? If I type a filename in the picker afterward and hit Save, does it save the file even though I closed the tab?
Just like webpages in browser can capture its closing and ask you if you really want to quit.
Same with your proposed workflow, when you try to close the tab, the file picker will pop up again (within the visible area of the tab, not on another monitor), tab stays opened.
