GNU bug report logs - #19194
24.4.50; `window-body-width' is not dynamic relative to font size changes

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Message #29 received at 19194 <at> debbugs.gnu.org (full text, mbox):

From: martin rudalics <rudalics <at> gmx.at>
To: Joe Corneli <holtzermann17 <at> gmail.com>
Cc: 19194 <at> debbugs.gnu.org
Subject: Re: bug#19194: 24.4.50; `window-body-width' is not dynamic relative
 to font size changes
Date: Thu, 27 Nov 2014 19:35:25 +0100

> I think you are totally right.  To keep the buffer and window
> distinction properly, my note should probably be read as a feature
> request, not a bug report.  The request is for a function such as
> `buffer-body-width' that would return the width of the current displayed
> buffer in em-length units.

What is the width of a buffer?  What are em-lenght units?

> Sounds promising!  I just pressed C-x C-- which runs `text-scale-adjust'
> to the effect: "Decrease the default face height by one step".

That's incorrect.  The doc-string of `text-scale-decrease' tells it more
accurately: "Decrease the height of the default face in the current
buffer by DEC steps."

> The step
> is `text-scale-mode-step', unchanged from its default value of 1.2.  The
> number of steps looks to be stored buffer-locally as
> `text-scale-mode-amount'.
>
> ... So a candidate function would be:
>
> (defun buffer-body-width (&optional buffer pixelwise)
>    (let ((width (window-body-width (get-buffer-window (or buffer
> 							 (current-buffer)))
> 				  pixelwise)))
>      (floor (cond
> 	    ((eq text-scale-mode-amount 0)
> 	     width)
> 	    ((> text-scale-mode-amount 0)
> 	     (/ width (* text-scale-mode-step text-scale-mode-amount)))
> 	    ((< text-scale-mode-amount 0)
> 	     (* width (* -1 text-scale-mode-step text-scale-mode-amount)))))))

We could start from here.  But:

(1) `text-scale-mode-amount' is not autoloaded, so we get an error
    calling this with emacs -Q.

(2) `text-scale-mode-amount' is buffer-local.  So we have to choose the
    right buffer before evaluating it.

(3) `text-scale-mode-amount' constitutes a request to the display engine
    to scale a face height.  What shall we do when our target machine
    can't display the character with the requested height and uses, for
    example, the nearest available height instead?

(4) I don't know whether and how the frame's `font' parameter can/should
    affect the height of the "default face".  Likely this is not a
    problem - Eli will tell.

As I said before, I'd rather have a buffer-local equivalent of the
variable `frame-char-height', something like `buffer-char-height',
instead of having to find out by myself what the correct value is.

Next we should try to incorporate this in `window-body-height', either
by overloading the PIXELWISE argument - for example, if this is the
symbol `lines-scaled' we'd return the scaled lines - or with an extra
BUFFER argument which would also allow to retrieve the body height of a
window as if it displayed BUFFER or with something better yet ...

As a consequence, we'd probably have to rename the current C routine
`window-body-height' to `window-body-height-internal' and write the new
`window-body-height' in Elisp on top of that.

And finally we would have to do that for all related functions like
`window-total-height', `split-window' or `window-resize' and decide how
a user can specify that, when splitting a window via say C-4 C-x 2, the
top window should have four lines counted in the original window
buffer's text scaling.

martin

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