Prevent stale content in Fast Render Mode

We do that by re-render visited pages that is not already in the stack. This may potentially do some double work, but that small penalty should be well worth it.

Fixes #5281

hugolib/site_render.go

@@ -43,7 +43,7 @@ func (s *Site) renderPages(cfg *BuildCfg) error {
 		go pageRenderer(s, pages, results, wg)
 	}
 
-	if len(s.headlessPages) > 0 {
+	if !cfg.PartialReRender && len(s.headlessPages) > 0 {
 		wg.Add(1)
 		go headlessPagesPublisher(s, wg)
 	}