bug#45556: 27.1; Unexpected behavior of `tab-bar-show' variable

bug#45556: 27.1; Unexpected behavior of `tab-bar-show' variable

[Marcel Ventosa]

Hello,

I may be misunderstanding the purpose of the `tab-bar-show' variable.


Expected behavior
=================

If tab-bar-show = 1, Tab Bar is not visible when less than two tabs are
available.

If tab-bar-show = nil, Tab Bar is never visible

If tab-bar-show = t, Tab Bar is always visible

In addition, I would like to be able to change the value of
tab-bar-show to toggle the visibility of the Tab Bar (I don't want to
disable the mode, only hide the Tab Bar).


Actual behavior and recipe
==========================

Run "emacs -q", switch to *scratch* and evaluate the following, in
order:

(tab-bar-mode)
;; tab bar is shown on top of buffer
(setq tab-bar-show 1)
;; no change. Maybe we need to toggle the mode to effect changes?
(tab-bar-mode 0)
;; bar disappears
(tab-bar-mode)
;; bar still reappears with only one tab
;; expected behavior: tab bar remains hidden because only one tab exists
(tab-bar-new-tab)
;; two tabs exist and are visible
(tab-bar-close-tab)
;; only one tab is left, tab bar is hidden
(tab-bar-mode)
;; turn mode off again
(setq tab-bar-show nil)
;; no bar should be visible if I turn tab-bar-mode on again
(tab-bar-mode)
;; tab bar is still visible even with the variable set to nil


Best regards,

Marcel


In GNU Emacs 27.1 (build 1, x86_64-pc-linux-gnu, GTK+ Version 3.24.22,
cairo version 1.17.3) of 2020-08-29 built on juergen
Windowing system distributor 'The X.Org Foundation', version
11.0.12010000 System Description: Arch Linux

Recent messages:
For information about GNU Emacs and the GNU system, type C-h C-a.

Configured using:
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bug#45556: 27.1; Unexpected behavior of `tab-bar-show' variable

[Juri Linkov]

> (tab-bar-mode)
> ;; tab bar is shown on top of buffer
> (setq tab-bar-show 1)
> ;; no change. Maybe we need to toggle the mode to effect changes?

This is expected behavior because tab-bar-show is designed
to be customized using

  (customize-set-variable 'tab-bar-show 1)

> (tab-bar-mode 0)
> ;; bar disappears
> (tab-bar-mode)
> ;; bar still reappears with only one tab
> ;; expected behavior: tab bar remains hidden because only one tab exists

Actually the low-level function 'tab-bar-mode' was designed
to give more freedom to override the value of 'tab-bar-show'.
So you don't need to use 'tab-bar-mode' because
after customizing 'tab-bar-show', the commands
'tab-bar-new-tab' and 'tab-bar-close-tab' will do the right thing
as your next examples demonstrate:

> (tab-bar-new-tab)
> ;; two tabs exist and are visible
> (tab-bar-close-tab)
> ;; only one tab is left, tab bar is hidden

But below again, no need to use low-level tab-bar-mode,
and tab-bar-show should be changed with customize.

> (tab-bar-mode)
> ;; turn mode off again
> (setq tab-bar-show nil)
> ;; no bar should be visible if I turn tab-bar-mode on again
> (tab-bar-mode)
> ;; tab bar is still visible even with the variable set to nil

So there are two levels: at high level you can customize 'tab-bar-show',
and tab commands do the right thing.

If you want more control, then you can use 'tab-bar-mode'.
Or if you want to enable/disable the tab bar individually
on each new frame, then you can use such configuration:

  (add-hook 'after-make-frame-functions 'toggle-frame-tab-bar)

bug#45556: 27.1; Unexpected behavior of `tab-bar-show' variable

[Marcel Ventosa]

Hi Juri,

Thank you for your response! After your detailed explanation I was able
to set things up the way I want, though I still have a question below
inline. 

I was also unaware of the differences between setting a variable with
'setq' and 'customize-set-variable'. 

This was not a bug after all. But would it be useful to mention in the
doc string that the variable's behavior will be different if modified
by setq? Or is this already common knowledge? 


Best regards,

Marcel

On Tue, 05 Jan 2021 20:43:48 +0200
Juri Linkov <juri@linkov.net> wrote:

> So there are two levels: at high level you can customize
> 'tab-bar-show', and tab commands do the right thing.
> 
> If you want more control, then you can use 'tab-bar-mode'.
> Or if you want to enable/disable the tab bar individually
> on each new frame, then you can use such configuration:
> 
>   (add-hook 'after-make-frame-functions 'toggle-frame-tab-bar)

Somehow reading a function name "toggle-tab-bar-mode..." my
understanding is that such a function would enable and disable tab-bar
functionality itself, rather than only show and hide the bar. I see
now that toggling the 'tab-bar-mode' minor mode also does not
enable/disable the tab-bar functionality, but rather seems to just
toggle the actual bar. Is this correct? If so, what is the difference
between 'toggle-tab-bar-mode-from-frame' and 'tab-bar-mode'. Both of
them seem to toggle the tab bar in _all_ frames.

bug#45556: 27.1; Unexpected behavior of `tab-bar-show' variable

[Juri Linkov]

[-- Attachment #1: Type: text/plain, Size: 456 bytes --]

> This was not a bug after all. But would it be useful to mention in the
> doc string that the variable's behavior will be different if modified
> by setq? Or is this already common knowledge? 

Thank you for the suggestion to mention this in the doc string.
While updating the doc string, I noticed the problem that currently
it doesn't update each frame individually according to the new
customized value, so this problem will be fixed with this patch:


[-- Warning: decoded text below may be mangled, UTF-8 assumed --]
[-- Attachment #2: tab-bar-show.patch --]
[-- Type: text/x-diff, Size: 1534 bytes --]

diff --git a/lisp/tab-bar.el b/lisp/tab-bar.el
index 5a95e5975d..69746d5f84 100644
--- a/lisp/tab-bar.el
+++ b/lisp/tab-bar.el
@@ -236,18 +236,27 @@ tab-bar-show
 and show it again once more tabs are created.
 If nil, always keep the tab bar hidden.  In this case it's still
 possible to use persistent named window configurations by relying on
-keyboard commands `tab-new', `tab-close', `tab-next', `tab-switcher', etc."
+keyboard commands `tab-new', `tab-close', `tab-next', `tab-switcher', etc.
+
+Please customize this variable using the Customization UI, then
+it will automatically update the existing tab bars on each frame."
   :type '(choice (const :tag "Always" t)
                  (const :tag "When more than one tab" 1)
                  (const :tag "Never" nil))
   :initialize 'custom-initialize-default
   :set (lambda (sym val)
          (set-default sym val)
-         (tab-bar-mode
-          (if (or (eq val t)
-                  (and (natnump val)
-                       (> (length (funcall tab-bar-tabs-function)) val)))
-              1 -1)))
+         ;; Preload button images
+         (tab-bar-mode 1)
+         ;; Then handle each frame individually
+         (dolist (frame (frame-list))
+           (set-frame-parameter
+            frame 'tab-bar-lines
+            (if (or (eq val t)
+                    (and (natnump val)
+                         (> (length (funcall tab-bar-tabs-function frame))
+                            val)))
+                1 0))))
   :group 'tab-bar
   :version "27.1")
 

[-- Attachment #3: Type: text/plain, Size: 1634 bytes --]


> Somehow reading a function name "toggle-tab-bar-mode..." my
> understanding is that such a function would enable and disable tab-bar
> functionality itself, rather than only show and hide the bar. I see
> now that toggling the 'tab-bar-mode' minor mode also does not
> enable/disable the tab-bar functionality, but rather seems to just
> toggle the actual bar. Is this correct?

This is almost correct.  Additionally, 'tab-bar-mode' also
loads button images and assigns C-TAB keys.

> If so, what is the difference between 'toggle-tab-bar-mode-from-frame'
> and 'tab-bar-mode'. Both of them seem to toggle the tab bar in
> _all_ frames.

Indeed, 'tab-bar-mode' toggles the tab bar in all frames, but
'toggle-tab-bar-mode-from-frame' is a very specialized command.
It's used only in the Show/Hide menu, to have the toggle reflect
the current frame, as the comment before its definition says.
Maybe this comment should be moved to its doc string like this:

diff --git a/lisp/tab-bar.el b/lisp/tab-bar.el
index 5a95e5975d..e239a02a64 100644
--- a/lisp/tab-bar.el
+++ b/lisp/tab-bar.el
@@ -187,9 +187,9 @@ tab-bar-handle-mouse
         ;; Clicking anywhere outside existing tabs will add a new tab
         (tab-bar-new-tab)))))
 
-;; Used in the Show/Hide menu, to have the toggle reflect the current frame.
 (defun toggle-tab-bar-mode-from-frame (&optional arg)
   "Toggle tab bar on or off, based on the status of the current frame.
+Used in the Show/Hide menu, to have the toggle reflect the current frame.
 See `tab-bar-mode' for more information."
   (interactive (list (or current-prefix-arg 'toggle)))
   (if (eq arg 'toggle)

bug#45556: 27.1; Unexpected behavior of `tab-bar-show' variable

[Juri Linkov]

tags 45556 fixed
close 45556 28.0.50
thanks

>> This was not a bug after all. But would it be useful to mention in the
>> doc string that the variable's behavior will be different if modified
>> by setq? Or is this already common knowledge? 
>
> Thank you for the suggestion to mention this in the doc string.

Now all updates were pushed to master, and this bug report is closed.