By analogy to the mode-line-inactive face, I'd like to dim the header line for inactive windows. But I can't find a "header-line-inactive" face, nor can I find an appropriate hook to modify.

I have the following in my init file, so every window gets a header line.

(setq-default header-line-format 
                (vc-mode vc-mode)
  • Looks like you would need to set the face each time you switch windows. You could do this by advising select-window, or perhaps using a buffer-list-update-hook. – glucas Nov 14 '14 at 1:26
  • Could you specify the header-line-format you use? (I need to know it to be able to improve my answer.) – Constantine Nov 14 '14 at 6:30
  • I didn't know header lines were supported. There's always something new to discover with Emacs! – glucas Nov 14 '14 at 14:38

You can set the face of the header line by setting properties of the text in header-line-format, but you would need to update header-line-format on every window switch.

As mentioned by @Slomojo, buffer-list-update-hook is the hook to use.

You can start with this draft of a hook and adjust the format string and text properties:

(defun my-update-header ()
   (lambda (window)
     (with-current-buffer (window-buffer window)
       (if (eq window (selected-window))
           (setq header-line-format (propertize "selected" 'face 'mode-line))
         (setq header-line-format (propertize "inactive" 'face 'mode-line-inactive)))))

(add-hook 'buffer-list-update-hook #'my-update-header)

Update Nov 14, 2014:

Here's an extension of the idea used above to arbitrary header formats:

(defun my-update-header ()
   (lambda (window)
     (with-current-buffer (window-buffer window)
       ;; don't mess with buffers that don't have a header line
       (when header-line-format
         (let ((original-format (get 'header-line-format 'original))
               (inactive-face 'warning)) ; change this to your favorite inactive header line face
           ;; if we didn't save original format yet, do it now
           (when (not original-format)
             (put 'header-line-format 'original header-line-format)
             (setq original-format header-line-format))
           ;; check if this window is selected, set faces accordingly
           (if (eq window (selected-window))
               (setq header-line-format original-format)
             (setq header-line-format `(:propertize ,original-format face ,inactive-face)))))))

I save the original value of header-line-format as a property of header-line-format so it can be restored later. To change the face of inactive header lines I use :propertize.

This breaks if a buffer is displayed by more than one window, and I am convinced that there is no way to do this right: you can't set different buffer-local values of header-line-format in the selected window and the rest of windows displaying the same buffer. (The mode line gets special treatment in the Emacs redisplay code implemented in C.)

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  • Whoa. buffer-list-update-hook is rocking my world. I want to accept this answer, but first could you please clarify: Is there a way to make this work with arbitrary mode line constructs in header-line-format? I've tried and am failing thus far. – purple_arrows Nov 14 '14 at 5:41
  • I think I found it on my own. Instead of changing the string properties, I can use face-remap-add-relative / face-remap-remove-relative in the hook. I don't have it working yet, but it might do. I'm accepting this one on the assumption that I can come back later and edit it to use those functions – purple_arrows Nov 14 '14 at 6:25
  • Sorry I had a busy day! I was going to suggest keeping a inactive header line alist and copy it to the inactive buffers. And an active header line a list you copy to the active buffer. I'd need to actually try a couple of things to be sure! - nice answer btw – ocodo Nov 14 '14 at 11:46

As a fairly incomplete answer (I'm in the bus so I'll flesh it out in a little while.)

You can use the buffer-list-update-hook This hook runs whenever the buffer list changes.

You can then determine which buffer is now current and update the header-line-format accordingly.

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