see martin-g's comments on
It seems there is some bug in checkboxx.js ver.1.3.0 (part of Wicket Bootstrap 0.9.12).
The bug affects only the "two state" mode of the widget.
Debugging its JS code I've found that the bug is not there if the native UI is used. So I've pushed a commit to master branch with that workaround.
Once Wicket-Bootstrap 0.9.13 is released please merge branch "
ISIS-1214-three-state-checkbox" to master. It reverts the first change and upgrades jQuery-UI that is used for dragging the modal dialogs.