After setting GOBJECT_DEBUG=objects comparing the outputs
of the following program run with ITERATIONS set to 2 and 1:

#include <gtk/gtk.h>

#ifndef ITERATIONS
#define ITERATIONS 2
#endif

int
main (int   argc, 
      char *argv[])
{
  if (gtk_init_check (&argc, &argv))                    
    {
      for (gint i = 0; i < ITERATIONS; ++i)             
        {
          GtkWidget *dialog = gtk_app_chooser_dialog_new_for_content_type (NULL, 0, "text/plain");

          gtk_widget_show (dialog);                     

          gtk_widget_destroy (dialog);                  
        }
    }

  return 0;
}


the number of objects alive at exit increases for the presence
of many (it depends on the number of suitable apps on the system)
GDesktopAppInfo objects referenced by GtkAppChooserWidget.

The problem is here:

https://git.gnome.org/browse/gtk+/tree/gtk/gtkappchooserwidget.c?h=gtk-3-22#n154

gtk_tree_model_get increments info's ref_count and this reference
is transferred to self->priv->selected_app_info in the two cases
where info is assigned to it, but it is not removed when
g_app_info_equal returns FALSE.