completion-in-region-mode-predicate is a variable defined in `minibuffer.el'.
Its value is
This variable may be risky if used as a file-local variable.
Predicate to tell `completion-in-region-mode' when to exit.
It is called with no argument and should return nil when
`completion-in-region-mode' should exit (and hence pop down
the *Completions* buffer).