Meaning of covered-buffers when splitting window

[mobid]

Hi,

If have next case:

1. Start in scratch buffer
2. Open two X windows, one above the other
3. Now when I split window I expect covered X window to be shown, instead scratch is displayed in other window.

As I saw in code https://github.com/emacs-exwm/exwm/blob/master/exwm-layout.el#L392 there is case which prevent my expected behavior. If I set there covered-buffers to nil it is working fine for me.

Is there any case when covered-buffers are meaningful?

[medranocalvo]

Hmmm I see.

(Context.) Usual Emacs behaviour when splitting is to display the same buffer in the new window. In EXWM we disallow an exwm-mode buffer to be displayed in more than one window. After splitting the window we notice that two windows display our exwm-mode buffer so we vacate one of them. I believe that the (selected-window) gets to keep it. Now, this new window must display another buffer; we go out of our way to prevent this other buffer to be an exwm-mode buffer being covered:

exwm/exwm-layout.el

Lines 381 to 390 in 2bb9a57

	;; Note any `exwm-mode' buffer is being covered by another
	;; `exwm-mode' buffer. We want to avoid that `exwm-mode'
	;; buffer to be reappear in any of the vacated windows.
	(let ((prev-buffer (car-safe
	(car-safe (window-prev-buffers window)))))
	(and
	prev-buffer
	(with-current-buffer prev-buffer
	(derived-mode-p 'exwm-mode))
	(push prev-buffer covered-buffers)))))))))

Is there any case when covered-buffers are meaningful?

I should know because I implemented this... but didn't write an explanation in ch11ng/exwm#145. Let us... reconstruct a reason.

Please, edit

exwm/exwm-layout.el

Line 392 in 2bb9a57

(let ((exwm-layout--other-buffer-exclude-buffers covered-buffers))

to have instead:

    (let ((exwm-layout--other-buffer-exclude-buffers nil))

Now perform the following steps:

1. Start in scratch buffer
2. Split vertically C-x 3
3. Split vertically C-x 3
4. Launch XEyes
5. Launch XTerm
6. Select middle window C-x o
7. Switch to XEyes C-x b XEyes - xeyes
8. Switch to XTerm C-x b XTerm - xterm

Why does XEyes suddenly display in the left window again? I just "moved" it to the middle one!

9. Select right window C-x o
10. Switch to XTerm C-x b XTerm - xterm

Shouldn't XEyes move back to the middle window, where I placed it?

---

I'm pretty sure that's the only reason I had for implementing this covered-buffers (mis?)feature. That is, although without covered-buffers our behaviour would be more similar to regular Emacs buffers, it kind of ends up being unintuitive. I think that something similar happens with your proposal above: will showing the prev-buffer on the split window match users' intuition?

(To clarify, I'm opening this for discussion. I'm not sure what's the better behaviour.)

---

Tangentially, I dislike that we are implementing this at all in EXWM: this should be a Display Buffer Action Function (https://www.gnu.org/software/emacs/manual/html_node/elisp/Buffer-Display-Action-Functions.html), e.g. display-buffer-unique-window. I'm not sure whether it is feasible. Also, this behaviour should in my opinion be optional: I believe that I would prefer having exwm-mode buffers displayed in multiple windows where only the selected one (or last selected) shows the actual X window and the others show, say, a caption or a screenshot (...). I'm also not sure whether this would be comfortable.

[mobid]

It make sense now. Maybe I didn't notice that, because I normally have only few X windows.

I'm fine with the both behaviours, which can be simply acomplished with checking last command in predicate:

(defun exwm-layout--other-buffer-predicate (buffer)
  (or (not (with-current-buffer buffer (derived-mode-p 'exwm-mode)))
      (and (not exwm-layout--other-buffer-exclude-exwm-mode-buffers)
           (or (not (memq buffer exwm-layout--other-buffer-exclude-buffers))
               (eq last-command #'split-window-below)
               (eq last-command #'split-window-right))
           ;; Do not select if already shown in some window.
           (not (get-buffer-window buffer t)))))

I understand that same X window cannot be shown in two places, but I rater see previous window instead of previous non-exwm buffer. (when splitting) Same split behaviour has StumpWM.

I believe that I would prefer having exwm-mode buffers displayed in multiple windows where only the selected one (or last selected) shows the actual X window and the others show, say, a caption or a screenshot

I like this idea.