Configure header line only if buffer major-mode does not display a header line

I've been using code taken mostly from the header line Emacs wiki to display the file path and current function of the current buffer.

(defmacro with-face (str &rest properties) `(propertize ,str 'face (list ,@properties))) (defun sl/make-header () "." (let* ((sl/full-header (abbreviate-file-name buffer-file-name)) (sl/header (file-name-directory sl/full-header)) (sl/drop-str "[...]") ) (if (> (length sl/full-header) (window-body-width)) (if (> (length sl/header) (window-body-width)) (progn (concat (with-face sl/drop-str :background "blue" :weight 'bold ) (with-face (substring sl/header (+ (- (length sl/header) (window-body-width)) (length sl/drop-str)) (length sl/header)) ;; :background "red" :weight 'bold ))) (concat (with-face sl/header ;; :background "red" :foreground "red" :weight 'bold))) (concat (if window-system ;; In the terminal the green is hard to read (with-face sl/header ;; :background "green" ;; :foreground "black" :weight 'bold :foreground "#8fb28f" ) (with-face sl/header ;; :background "green" ;; :foreground "black" :weight 'bold :foreground "blue" )) (with-face (file-name-nondirectory buffer-file-name) :weight 'bold ;; :background "red" ))))) (defun sl/display-header () "Create the header string and display it." ;; The dark blue in the header for which-func is terrible to read. ;; However, in the terminal it's quite nice (if window-system (custom-set-faces '(which-func ((t (:foreground "#8fb28f"))))) (custom-set-faces '(which-func ((t (:foreground "blue")))))) ;; Set the header line (setq header-line-format (list "-" '(which-func-mode ("" which-func-format)) '("" ;; invocation-name (:eval (if (buffer-file-name) (concat "[" (sl/make-header) "]") "[%b]"))) ) ) ) ;; Call the header line update (add-hook 'buffer-list-update-hook 'sl/display-header) 

However, this prevents major modes from setting their own header lines. For instance, when I use GDB with many-windows set to true, this prevents me from toggling between locals/registers and threads/breakpoints.

I tried changing the code block to

(if header-line-format nil (add-hook 'buffer-list-update-hook 'sl/display-header)) 

but this does not give the expected result (it seems to check for the first buffer and then not after that).

Is there a way to display my own custom header line only if the current buffer wouldn't display its own?