The default location is set to be the one configured in the
org.gtk.Settings.FileChooser/startup-mode GSettings key; the settings for the file chooser are loaded when the widget is mapped.
Whenever you switch to a different GtkNotebook page, the notebook will set the
child-visible property on the widget in the newly visible page to
TRUE, and in the old page to
FALSE. Changing the child visibility will cause a widget to be mapped or unmapped. This means that the file chooser will reload its settings every time you switch a page, thus resetting the current location.
You can connect to the
unmap signal on the file chooser widget to store the current location, and to the
map signal to set that location back.