Reference Manual - 0.9.25
IDirectFBSurface   GetVisibleRectangle

Created sub surfaces might be clipped by their parents, this function returns the resulting rectangle relative to this surface.

GetVisibleRectangle (
  IDirectFBSurface   * thiz,
  DFBRectangle   * ret_rect
);

For non sub surfaces this function returns { 0, 0, width, height }.