The layered nav filtering doesn't work well with variable products
when some variations have stock and other don't. When a term is
selected in the widget, a variable product having no stock for
the variation corresponding to that term but having stock for
other variations will be displayed, but it shouldn't.
This commit fixes that by introducing two changes:
- A new override of "is_visible" for WC_Product_Variable that
looks at the supplied filters, compares them against the corresponding
available variations and calculates the visibility based on
the query type (OR or AND).
- A hook on the "found_posts" filter in WC_Query, that adjusts
the posts count based on the found products visibility
when there are filters available; this is needed to sync the
"displaying X posts" messages and the paging when variable
products are hidden due to stock status.
Additionally, the visibility calculated in "found_posts" is cached
as loop variables so that it isn't calculated again when actually
displaying the products.
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