bug#67604: Motion problems with inline images

[JD Smith]

On Dec 6, 2023, at 7:25 AM, Eli Zaretskii <eliz@gnu.org> wrote:

From: JD Smith <jdtsmith@gmail.com>
Date: Tue, 5 Dec 2023 23:33:42 -0500
Cc: 67604@debbugs.gnu.org

Not really, no.  But reproducing the problem is just a step towards
debugging it, so the alternative is for you to step through next-line
and its subroutines

I stepped through next-line and subroutines via edebug and landed via line-move-visual on:

(vertical-motion  (cons (or goal-column
    (if (consp temporary-goal-column)
(car temporary-goal-column)
      temporary-goal-column))
arg))

So vertical-motion is where all fingers point.

Did you verify that goal-column and temporary-goal-column have correct
values in the case where the problem happens?

Yes.  Sometimes the temporary-goal-column is 0 (if I C-a on a text line) or sometimes a fractional number like 66.5 or similar if I start my downward movement by first C-a’ing on the green image line.  In both cases the short screen line #3 line is skipped.

It isn't like I'm the only
one who should be able to read the code and understand where it fails.

I agree with that, but unfortunately am not setup for it here and have next to no familiarity with Emacs’
C code.  I’m sorry I can’t be of more help.  But similar to how you were unable to work with dvisvgm
and other packages, I don’t have access to gdb, as it is not supported on my architecture.

If someone can reproduce and debug the problem on a system other than
macOS, that would be some progress.  (If the problem is specific to
macOS, it is much less interesting, at least to me, since the display
code on macOS behaves differently in significant ways, and because
macOS is in general an idiosyncratic platform.)

I have made progress.  I built master on a remote Linux server and ran it via X forwarding.  With an 8x17 font, the bug is found at offset 2 = 148 pixel width of the red SVG, for the specific frame-width = 74.   I verified that (vertical-motion 1) causes the same line-skip error, when starting from the green image.  

This makes me think that your difficulty reproducing the bug on your system may result from a miscommunication.  To be sure, I’ve recorded a session of me (re-)discovering the bug at this comment.  Hopefully that’s useful.  

If not, I did the build on my Linux server with debug options, and can run over gdb. If you can give me a simple recipe for where I should break and what I should look at, I’m happy to try.  But I suspect it will be 25x more efficient for you to have a look if you can reproduce.

Re MacOS, I’m surprised you wouldn’t say that Windows is similarly an idiosyncratic system for display and other purposes.