gdk:surface-set-opaque-region (surface region)

surface -- a toplevel or non-native gdk:surface object
region -- a cairo:region-t instance, or nil

Marks a region of surface as opaque. For optimization purposes, compositing window managers may prefer to not draw obscured regions of surfaces, or turn off blending for these regions. With RGB windows with no transparency, this is just the shape of the window, but with ARGB32 windows, the compositor does not know what regions of the window are transparent or not. This function only works for toplevel surfaces.

GTK will update this property automatically if the surface background is opaque, as we know where the opaque regions are. If your surface background is not opaque, please update this property in your GtkWidgetClass.css_changed() handler. This function is deprecated since 4.16. GDK can figure out the opaque parts of a window itself by inspecting the contents that are drawn.