GNU bug report logs - #78621
Pixelwise display specified spaces less precise than min-width

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Message #38 received at 78621 <at> debbugs.gnu.org (full text, mbox):

From: Eli Zaretskii <eliz <at> gnu.org>
To: Jim Porter <jporterbugs <at> gmail.com>
Cc: shipmints <at> gmail.com, 78621 <at> debbugs.gnu.org
Subject: Re: bug#78621: Pixelwise display specified spaces less precise than
 min-width
Date: Thu, 29 May 2025 09:18:48 +0300

> Cc: 78621 <at> debbugs.gnu.org
> Date: Wed, 28 May 2025 16:07:48 -0700
> From: Jim Porter <jporterbugs <at> gmail.com>
> 
>  From my read of this bug, the issue was originally that these snippets 
> should have the same display width:
> 
>    (concat "some text"
>            (propertize " " 'display
>                        `(space :width (,(- width width-of-some-text)))))
> 
>    (propertize "some text" 'display `(min-width ((,width))))
> 
> I agree with that, but only if 'width-of-some-text' is computed correctly.
> 
> For variable-pitch text, each character cell (I hope I'm using the right 
> term here) in the display is a different width, rounded to the nearest 
> integer, which means that computing the pixel-width as in your original 
> reproducer accumulates small rounding errors for every character.

I don't follow this argument.  string-pixel-width uses the display
code, so it should return the exact same value in pixels as what the
actual display produces when the same characters are shown on the
screen.  So whatever rounding happens (which I don't think it does,
since font glyphs have integer advance width), it happens the same in
both cases and should yield the same values.  Or what am I missing?

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