fix(markup): skip post-processing inside MathML (#39497)

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

modules/markup/html.go

@@ -349,8 +349,8 @@ func visitNode(ctx *RenderContext, procs []processor, node *html.Node) *html.Nod
 		// TextNode emoji will be converted to `<span class="emoji">`, then the next iteration will visit the "span"
 		// if we don't stop it, it will go into the TextNode again and create an infinite recursion
 		return node.NextSibling
-	} else if node.Data == "code" || node.Data == "pre" {
-		return node.NextSibling // ignore code and pre nodes
+	} else if node.Data == "code" || node.Data == "pre" || node.Data == "math" {
+		return node.NextSibling // ignore code, pre and math nodes
 	} else if node.Data == "img" {
 		return visitNodeImg(ctx, node)
 	} else if node.Data == "video" {

modules/markup/html_test.go

@@ -543,6 +543,9 @@ func TestPostProcess(t *testing.T) {
 		`Some text with <span class="emoji" data-alias="smile">😄</span> in the middle`)
 	test("http://localhost:3000/person/repo/issues/4#issuecomment-1234",
 		`<a href="http://localhost:3000/person/repo/issues/4#issuecomment-1234" class="ref-issue">person/repo#4 (comment)</a>`)
+	test(
+		"<math><mtext>:gitea: go-gitea/gitea#12345</mtext></math>",
+		"<math><mtext>:gitea: go-gitea/gitea#12345</mtext></math>")
 
 	// special tags, GitHub's behavior, and for unclosed tags, output as text content as much as possible
 	test("<script>a", `&lt;script&gt;a`)