How do I show a multi-line sticky header in a buffer?

Question

I want to display a few lines at the top of a specific buffer, no matter how much the user has scrolled down (basically, a multi-line version of a header line). Moreover, I want these lines to be visible if and only if that buffer is visible (e.g. if I switch to a different buffer, the lines should disappear. When I switch back, they should be visible again).

(The specific use case I have is a weekly calendar view. The user can scroll to see different hours of the day, but I want the days of the week, all-day events, navigation buttons, etc. to always be visible at the top.)

So far the closest I've got is using a posframe (which uses child frames). This can successfully display such a "multi line header", but the problem I ran into is when killing/changing buffers the posframe does not disappear. The posframe appears to be associated with the whole frame, rather than a specific buffer. I have not found a way to have a posframe appear if and only if a specific buffer is visible.

I would appreciate help on either making the posframes work, or an alternative method for a multi-line sticky header. Thanks!

Answer 1

I think a good way to 'fake' this is to use an atomic window and set the window parameters mode-line-format to none and no-other-window to t as follows:

(let* ((buf "*multi-line-header")
       (win (display-buffer-in-atom-window
            (get-buffer-create buf)
            '((side . above) (window-height . 3)))))
  (set-window-parameter win 'no-other-window t)
  (set-window-parameter win 'mode-line-format 'none)
  (with-current-buffer buf
    (insert (propertize "This is a fake\nmulti-line\nheader" 'face "warning"))))

I have simply modified the example for using display-buffer-in-atom-window. However, its docstring mentions that it should only be called by display-buffer or a similar function. So you might want to read about that (I guess it would be cleaner, but that it is not essential).

Also, you would have to find a way to automatically show the header-window, when switching to your 'base' buffer (I guess you could find some hook to use for that), and delete it when you switch buffer in your 'base' window. But I hope this anwer gets you on the way...

(As you still have to create and delete the window when switching buffers, I am not sure if it has to be an atomic window at all, it just 'feels' right. So you should still think about if this is really necessary)