The display has changed
the Gtk.Settings.gtk_xft_dpi setting has changed
The font configuration has changed in a way that requires text to be redrawn. This can be any of the Gtk.Settings.gtk_xft_antialias, Gtk.Settings.gtk_xft_hinting, Gtk.Settings.gtk_xft_hintstyle, Gtk.Settings.gtk_xft_rgba or Gtk.Settings.gtk_fontconfig_timestamp settings
The Gtk.Settings.gtk_font_name setting has changed
The icon theme has changed in a way that requires icons to be looked up again
Values that can be passed to the Gtk.Widget.system_setting_changed vfunc.
The values indicate which system setting has changed. Widgets may need to drop caches, or react otherwise.
Most of the values correspond to Settings properties.
More values may be added over time.