I saw this custom component in use at a past employer and thought it would be something useful to add to my toolbox. Not wanting to steal their code (and not caring for the way it was implemented either) I rolled my own version.
It's a CheckBox component with an additional "schrodinger" state. You see them used a lot in data trees where some, but not all, children of a node are selected, and therefore, the parent should be partially selected.
The first example, below, shows the component by itself. It can be set to its schrodinger state by setting "schrodinger = true".
Below that is an example of it used in a data tree.
Source code located at: https://www.assembla.com/code/shinynet/subversion/nodes