which-func-non-auto-modes

which-func-non-auto-modes is a variable defined in `which-func.el'.
Its value is

nil

Documentation:
List of major modes where Which Function mode is inactive till Imenu is used.
This means that Which Function mode won't really do anything
until you use Imenu, in these modes. Note that files
larger than `which-func-maxout' behave in this way too;
Which Function mode doesn't do anything until you use Imenu.

You can customize this variable.