// application/x-www-form-urlencoded, the request Body is not read, and
// r.PostForm is initialized to a non-nil, empty value.
//
// [Request.ParseMultipartForm] calls ParseForm automatically.
// ParseForm is idempotent.
func (r *Request) ParseForm() error {
	var err error
	if r.PostForm == nil {
		if r.Method == "POST" || r.Method == "PUT" || r.Method == "PATCH" {
			r.PostForm, err = parsePostForm(r)
		}

		nonce        string // a unique per-request string
		sawSlowNonce bool   // true if the handler saw /slow?nonce=<nonce>
	)
	cst := newClientServerTest(t, mode, HandlerFunc(func(w ResponseWriter, r *Request) {
		_ = r.ParseForm()
		if r.URL.Path == "/" {
			Redirect(w, r, "/slow?nonce="+r.Form.Get("nonce"), StatusFound)
			return
		}
		if r.URL.Path == "/slow" {
			mu.Lock()
			if r.Form.Get("nonce") == nonce {