Alt+P used to do this in the gtk2 filechooser (well, sometimes). Add this back as a key binding.
https://bugzilla.gnome.org/show_bug.cgi?id=720684
826c1db file chooser: Add a shortcut to focus the sidebar
gtk/gtkfilechooserwidget.c | 33 +++++++++++++++++++++++++++++++++
1 file changed, 33 insertions(+)
Upstream: git.gnome.org