How to close a workspace?

I have a project which upon opening always re-opens 4 workspaces (with 2015r4.1, OSX)

I tried to close 3 workspaces and then do some changes within the last remaining IDE window and then close that project.

But when re-opening it, then I again have 4 workspaces.

How does this actually work?

P.S.: I still wished there was some kind of workspace management available in the IDE:

It’s a bug in 2015r4 and has been fixed for the next release.

The temporary fix is to delete the hidden uistate file that sits next to the project.