Thanks for checking this, Manuel.  I’ve already filed a bug report and Eli has found a fix, but unfortunately only for this very specific case with line truncation.  The general issue remains.  We are seeking a self-contained reproducer for the more general case in other wrap modes (visual-line, normal wrapping).  

Please see https://debbugs.gnu.org/cgi/bugreport.cgi?bug=67533 to continue the discussion there.

> On Dec 1, 2023, at 3:36 AM, Manuel Giraud <manuel@ledu-giraud.fr> wrote:
> 
> JD Smith <jdtsmith@gmail.com <mailto:jdtsmith@gmail.com>> writes:
> 
>> In developing a new pixel-precise smooth scrolling mode, I’ve noticed
>> that inline SVG images confuse Emacs regarding pixel positions of
>> surrounding elements.  You sometimes also experience this while
>> visiting SVG image-rich files (think org-latex-preview) while in
>> visual line mode.  In this case, previous/next-line sometimes jump
>> from one side of the window to the other.
>> 
>> But it’s easiest to reproduce with line-truncation in effect.  Run the
>> snippet below with your frame either expanded wide enough to
>> accommodate the full width of the 1st line of text, or too narrow
>> (eliciting truncation).  While truly truncated and with point on the
>> SVG, pixel text measurements above are erroneous (reporting zero pixel
>> height above), as if it thinks it’s on the prior line.  At other
>> points in line 2, the pixel-size values are correct.
> 
> Hi, I can reproduce this on master.  Below I modified your example
> slightly and I also observe different pixel width for the same text
> line.
> 
> I think you should file a bug report with this issue.  Thanks.
> 
> --8<---------------cut here---------------start------------->8---
> ;;; test-svg-pixel-position --- test pixel position for SVG images
> 
> ;;; This small code creates a buffer with two lines, the first of
> ;;; which is long, and the second of which has an SVG image at start.
> ;;; Line truncation is turned on.  `window-text-pixel-size` returns
> ;;; differing results depending on whether truncation is actually in
> ;;; effect (alter the frame width to see this).
> 
> ;;; Code:
> (require 'svg)
> (let ((buf "svg-pixel-demo")
>      (svg (svg-create 50 25)))
>  (svg-circle svg 25 25 25 :stroke-color "green")
>  (with-current-buffer (get-buffer-create buf)
>    (erase-buffer)
>    (insert "Pellentesque condimentum, magna ut suscipit hendrerit, ipsum augue ornare nulla, non luctus diam neque sit amet urna.\n")
>    (insert "Pellentesque condimentum, magna ut suscipit hendrerit, ipsum augue ornare nulla, non luctus diam neque sit amet urna.\n")
>    (insert (propertize "THISISACIRCLE" 'display (svg-image svg)))
>    (insert " Aliquam posuere.\n")
>    (pop-to-buffer buf)
>    (goto-char (point-max))
>    (forward-line -1)
>    (toggle-truncate-lines 1)
>    (let ((above-image (window-text-pixel-size nil (cons (point) -1) (point) nil nil nil t)))
>      (forward-line -1)
>      (message "FIRST LINE: %S; ABOVE IMAGE: %S"
> 	       (window-text-pixel-size nil (cons (point) -1) (point) nil nil nil t)
> 	       above-image))))
> --8<---------------cut here---------------end--------------->8---
> -- 
> Manuel Giraud