AllowGroupClose

AllowGroupClose
is an option for Cell that specifies whether a cell group can be closed normally.

Closing a group collapses all cells in the group so that only the head cell is visible.

With the default setting AllowGroupClose->Automatic, any cell group can be closed normally.

Create a notebook where any cell group can be closed normally:

The cell group can be closed normally:

Create a notebook where the "heading" cell group cannot be closed:

Now the "heading" cell group cannot be closed:

Create a notebook where any cell group can be closed normally:

The cell group can be closed normally:

Create a notebook where the "heading" cell group cannot be closed:

Now the "heading" cell group cannot be closed: