For me it reproducible 100% with following scenario:
First open a project and some files in that project.
Than in the windows explorer "start" another project. I did configured CB that there is only one instance of CB may be started, and associated the CB projects files with the CB.
With that scenario CB always splits the Editor and in that way, that in one side there are files form the first project, in other side - from second project. I use the revision 10554
The problem of that behavior is that I can not unsplit it, and in one side of the split there are more than one files, and it makes some difficulties on small monitors, f.e. on notebook.
See screenshot here (sorry for small download of it)