>> So do you propose to prefer buffer-local next-error-last-buffer
>> instead of window-local next-error-last-buffer?
>
> Maybe something like this:
>
> - Make next-error-last-buffer always buffer-local.

I believe this patch is a change for the better:

1. makes next-error-function buffer-local

2. sets both buffer-local and global values

3. adds customizable next-error-find-buffer-function

4. moves window-on-frame-visibility code to separate function
   that can be used to customize for backward-compatibility

5. adds next-error-select-buffer to manually switch to another
   next-error capable buffer

6. message to show which next-error buffer is currently used