(apparently lost, so rewritten and reposted) Re: header-line-format hacking

[ytrewq1 <ytrewq1@gmail.com>]

Thien-Thi Nguyen writes:

> the minor mode code presuambly comprises a bunch of defuns
> plus some "load-time actions".  the load-time actions is
> where the variable is inserted into `header-line-format'
> (be sure to check that it's not already there).
> 
> the idea is to modify `header-line-format' at most one time,
> and use indirection (through the variable) to effect state
> toggling and notification.

Thanks for the explanation.

I think I understand how this approach could work for
mode-line-format as it's default value is non-nil.  However,
on my system, header-line-format has a default value of
nil.  As far as I can tell, if a buffer's header-line-format 
value is modified to be non-nil a header is displayed -- so
if a buffer starts out w/ a header-line-format value of nil
and minor mode code modifies it to be non-nil, deactivation
of the minor mode will cause the buffer to have an empty
header line ("baggage").

Is there a way to avoid this "baggage" using the idiomatic
approach?