Hi Darran,
Thanks for the patience.
We have created a sample to illustrate DockstateChanging Event in DockingManager using Prism which you can download under the following location.
In this sample, DockingManager has a DockstateChanging Event which fires before the Dockstate is changed. Using this Handler we can perform the cleaning up operation before the state is being changed. After this handler, DockStateChanged Event will be fired and here we can remove the complete element or region.
Please let me know if the sample helps.
Regards
Surya Narayana
CS_91972e44.zip