Newer GTK windows using client-side decorations use extra geometry around their frames for effects like shadows and invisible borders.
Window managers that want to maximize windows or snap to edges need to know where the extents of the actual frame lie, so that users don’t feel like windows are snapping against random invisible edges.
Note that this property is automatically updated by GTK, so this function should only be used by applications which do not use GTK to create toplevel surfaces.
The left extent
The right extent
The top extent
The bottom extent