When used, Rectangle component is shown in front of most other components in Workflow Designer. Some other components as well as flow paths are shown in front of it. This prevents from easily seeing or selecting the covered components.
Some properties for the Rectangle component are not functional, for example Background Color.
This issue is fixed in Workflow/ServiceDesk 8.1 RU6.
There are no known workarounds except removing the Rectangle component which is complicated due to related issue of Rectangle component not being selected properly by clicking on it.