You cannot rejoin a partition if either the project or any of the modules in the partition are open.
Note: In the database explorer, the folder or project that is selected in the left pane is open and has an open folder icon

or an open project icon

. Its parent folder or project is also open, and so are all the folders and projects above it in the database tree, although they do not have open folder or open project icons.
When a module is open, all the folders and projects above it in the database tree are also open.