Fix shortcode detection in RenderString

Fixes #10654

hugolib/page__per_output.go

@@ -384,8 +384,8 @@ func (p *pageContentOutput) RenderString(args ...any) (template.HTML, error) {
 
 	var rendered []byte
 
-	if strings.Contains(contentToRender, "{{") {
-		// Probably a shortcode.
+	if pageparser.HasShortcode(contentToRender) {
+		// String contains a shortcode.
 		parsed, err := pageparser.ParseMain(strings.NewReader(contentToRender), pageparser.Config{})
 		if err != nil {
 			return "", err

hugolib/renderstring_test.go

@@ -190,3 +190,39 @@ Has other: false
 `)
 
 }
+
+func TestRenderStringWithShortcodeIssue10654(t *testing.T) {
+	t.Parallel()
+
+	files := `
+-- config.toml --
+timeout = '300ms'
+-- content/p1.md --
+---
+title: "P1"
+---
+{{< toc >}}
+
+## Heading 1
+
+{{< noop >}}
+     {{ not a shortcode
+{{< /noop >}}
+}
+-- layouts/shortcodes/noop.html --
+{{ .Inner | $.Page.RenderString }}
+-- layouts/shortcodes/toc.html --
+{{ .Page.TableOfContents }}
+-- layouts/_default/single.html --
+{{ .Content }}
+`
+
+	b := NewIntegrationTestBuilder(
+		IntegrationTestConfig{
+			T:           t,
+			TxtarString: files,
+		},
+	).Build()
+
+	b.AssertFileContent("public/p1/index.html", `TableOfContents`)
+}