>> Fwiden in the functions above (which are AFAICS the only places in 
>> Emacs where Fwiden is called) is not prepared to the possibility of 
>> them being called inside a labeled narrowing, either one installed by 
>> the long lines code, or another one.  Basically we need to use a 
>> variant of reset_outermost_narrowings (for the current buffer only) 
>> where we use
>>
>> record_unwind_protect (save_restriction_restore, save_restriction_save ());
>
> Ah, okay.  Please do, and thanks.
>

And here's the patch!