01 October 2018
I see what you are saying:
When you click on the pin, it should toggle between the Autohide modes:
Can you try double clicking on the little blue bar that says Outline. I just tried that and it docked the window for me.
If this doesn't work, go to Workbench and try 'Reset Workspace' and 'Reset Window Layout', then re-start WB and see if this fixes it?
It that doesn't work as well and if you are using Windows, close WB, open up file explorer and reset your appdata:
and rename the folder of the version you are using, looks like v191 to v191.old
Hopefully, this fixes the problem.