GtkOverlay is a container which contains a single main child, on top of which it can place “overlay” widgets.
The position of each overlay widget is determined by its halign and valign properties. E.g. a widget with both alignments set to gtk_align_start will be placed at the top left corner of the GtkOverlay container, whereas an overlay with halign set to gtk_align_center and valign set to gtk_align_end will be placed a the bottom edge of the GtkOverlay, horizontally centered. The position can be adjusted by setting the margin properties of the child to non-zero values.
More complicated placement of overlays is possible by connecting to the get_child_position signal.
GtkOverlay as GtkBuildable
The GtkOverlay implementation of the GtkBuildable interface supports placing a child as an overlay by specifying “overlay” as the “type” attribute of a `<child>` element.
GtkOverlay has a single CSS node with the name “overlay”. Overlay children whose alignments cause them to be positioned at an edge get the style classes “.left”, “.right”, “.top”, and/or “.bottom” according to their position.