Fix it so the owning taxonomy gets rerendered in server when new tags are added

Updates #13648

hugolib/content_map_page.go

@@ -1330,6 +1330,7 @@ func (h *HugoSites) resolveAndResetDependententPageOutputs(ctx context.Context,
 				// This needs no reset, so no need to check it.
 				return nil
 			}
+
 			// First check the top level dependency manager.
 			for _, id := range changes {
 				checkedCounter.Add(1)
@@ -1645,6 +1646,8 @@ func (sa *sitePagesAssembler) assembleTermsAndTranslations() error {
 		views   = sa.pageMap.cfg.taxonomyConfig.views
 	)
 
+	rebuild := sa.s.h.isRebuild()
+
 	lockType := doctree.LockTypeWrite
 	w := &doctree.NodeShiftTreeWalker[contentNodeI]{
 		Tree:     pages,
@@ -1677,6 +1680,14 @@ func (sa *sitePagesAssembler) assembleTermsAndTranslations() error {
 					pi := sa.Site.Conf.PathParser().Parse(files.ComponentFolderContent, viewTermKey+"/_index.md")
 					term := pages.Get(pi.Base())
 					if term == nil {
+						if rebuild {
+							// A new tag was added in server mode.
+							taxonomy := pages.Get(viewName.pluralTreeKey)
+							if taxonomy != nil {
+								sa.assembleChanges.Add(taxonomy.GetIdentity())
+							}
+						}
+
 						m := &pageMeta{
 							term:     v,
 							singular: viewName.singular,

hugolib/hugo_sites_build.go

@@ -324,6 +324,14 @@ func (h *HugoSites) assemble(ctx context.Context, l logg.LevelLogger, bcfg *Buil
 		}
 	}
 
+	// Handle new terms from assemblePagesStep2.
+	changes = bcfg.WhatChanged.Drain()
+	if len(changes) > 0 {
+		if err := h.resolveAndClearStateForIdentities(ctx, l, nil, changes); err != nil {
+			return err
+		}
+	}
+
 	h.renderFormats = output.Formats{}
 	for _, s := range h.Sites {
 		s.s.initRenderFormats()

hugolib/rebuild_test.go

@@ -1865,3 +1865,30 @@ p1-content|
 	b.EditFileReplaceAll("content/p1/index.md", "p1-content", "p1-content-foo").Build()
 	b.AssertFileContent("public/p1/index.html", "p1-content-foo")
 }
+
+func TestRebuildEditTagIssue13648(t *testing.T) {
+	t.Parallel()
+
+	files := `
+-- hugo.toml --
+baseURL = "https://example.com"
+disableLiveReload = true
+-- layouts/all.html --
+All. {{ range .Pages }}{{ .Title }}|{{ end }}
+-- content/p1.md --
+---
+title: "P1"
+tags: ["tag1"]
+---
+
+`
+	b := TestRunning(t, files)
+
+	b.AssertFileContent("public/tags/index.html", "All. Tag1|")
+	b.EditFileReplaceAll("content/p1.md", "tag1", "tag2").Build()
+
+	// Note that the below is still not correct, as this is effectively a rename, and
+	// Tag2 should be removed from the list.
+	// But that is a harder problem to tackle.
+	b.AssertFileContent("public/tags/index.html", "All. Tag1|Tag2|")
+}