Ok, here’s what I tried:
As far as I know from the documentation, the portfolio page is a special single page with the special selected portfolio layout template.
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Right?
So I thought if I wanted to put some special content on this page above the portfolio list I could best do that with its own single page layout. Because the portfolio page is by definition a single page although it behaves like an archive page and displays multiple items (portfolio items). And I guess that the portfolio items are loaded by script, which is obvious when filtering.
So if I create an archive template for portfolio items, how can I address it with a URL?
I have now decoupled the Portfolio page from the General page layout so that it no longer affects the Portfolio page.
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Unfortunately, for some reason, the filter functionality no longer works and is not displayed.
So again, I don’t see how this would work with the archive layout if the portfolio page is actually a single page.