Search index as custom view for taxonomy term with hierarchy. Showing all children results in term parent in taxonomy menu Ok so title is pretty epic, but it goes like this:
Key elements: views, taxonomy_menu, taxonomy, search api, facets.

I created vocabulary "Products" (Drupal Commerce but I don't think it's important), and there term - "Chemistry" - which should be a root of tree of other more specific terms. I added some products. Next I enabled Taxonomy Menu and created a menu, so far so good, but I wanted to use search api, so I created a view of my search index with path:


  /taxonomy/term/%


That way I wanted to override view of my taxonomy, and be able to have facets in it. And it perfectly displays indexed products of terms that have them, but it fails to display all products of CHILDREN terms. So if I have my term "Chemistry" - after clicking it - I would like to be able to see every product of its children terms. I cannot see how can I do this with current approach, as it seems that I doesn't have contextual filters for that (as my view is displaying search index).

What can I do?
If I'm understanding correctly what you're asking, here are some tips.

Under your defined search index, go to Fields. If you have "Products" taxonomy defined, uncheck the taxonomy term, then go to the bottom and open "Add Related Fields" and add "Products", then save changes. When it reloads, you'll now have more taxonomy options. Check "Products >> All parent terms". You should now have additional facets for parent/child terms.

Use in conjunction with Facet API Pretty Paths for even more powerful usage. Take a look at one of my other response about how to use that.