I have read through all 11 pages of this and cannot find the solution to this. Has anyone found a method, or a link to the method, to do this?
Right now, checking more than one category in the checkbox option returns results related to ANY of those categories. I would like results matching ALL of the checked categories.
Apparently, this was native functionality earlier but now no longer works this way. Nor can I find a solution for modifying the code to do so.
I know that this is possible by hand coding somehow (haven't found that clearly yet, either). Many sites indicate how to display multiple categories in checkboxes for filtering. I cannot see how to restrict filtering to all categories selected (or, ideally, to have a choice between ANY or ALL in the menu itself).
RESEARCH TO DATE:
On this site, I have read this confusing thread, but the plus signs are present in the version I downloaded the day before, and this still doesn't lead to an ALL filtering: http://wordpress.org/support/topic/plugin-query-multiple-taxonomies-change-relation-from-and-to-or?replies=5
None of the other similar posts were addressed that I saw in a long pass through two years of questions.
Off this site, I also have read:
How to hand code multiple checkboxes - http://melandri.net/2011/12/31/multiple-categories-filter-in-wordpress-search/ - no info on how to restrict to ALL filtering.
How to hand code and restrict (don't know php, so every attempt at this led to errors and didn't return results from custom post categories): http://www.mogmachine.com/wordpress-search-by-multiple-categories/
How to hand code and restrict to specifically named categories in the code itself (this is the exact functionality, but we have dozens of categories that keep expanding, and it would be great for this to occur dynamically and draw in whatever categories are there):
I also tried the Multiple Category Selection Widget plug-in but could not get it to work with custom posts and categories.
Any help at all would be greatly appreciated!