GNU bug report logs -
#73600
31.0.50; Visual wrap prefix mode and image display
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Message #53 received at 73600 <at> debbugs.gnu.org (full text, mbox):
[Message part 1 (text/plain, inline)]
On 10/13/2024 11:51 AM, Eli Zaretskii wrote:
> That is probably correct, at least in some cases, but why should we
> rely on that for this purpose? Isn't it more reliable and
> future-proof to skip the entire span of the outermost "replacing"
> display property, and never look at properties inside that text? Does
> it really matter for visual-wrap-prefix-mode to look inside?
As I understand this bug, we just need to make sure that visual-wrap.el
doesn't change the display property of any single/atomic span of text
that xdisp.c will replace. Otherwise, xdisp.c will stop at every change
to 'display' in turn, leading to duplicated images (or whatever we're
displaying).
I think the most reliable/future-proof way to fix this would be work as
much like xdisp.c as we can. From what I can tell, xdisp.c is using
'next-single-char-property-change' for this: for
replacing-display-props, 'handle_single_display_spec' calls
'display_prop_end', which calls 'Fnext_single_char_property_change'.
That's what my patch is doing, though it's in Lisp, and it's erring on
the side of caution with what display specs could be "unsafe".
> IOW, I prefer simplicity of the logic to sophisticated design based on
> what we actually see Emacs do in tricky cases, because time and again
> I learned the hard way that my mental models of what actually happens
> can be erroneous in the fine details.
I've attached a patch that isn't *exactly* what you suggested, but
should add an extra layer of safety at the cost of possibly failing to
add wrapping properties in when we should/could. In this version
("be-extra-careful.patch"), whenever we find an unsafe display property
at the end of a line, we just skip ahead until we find the first
known-safe display property.
This is the best I could come up with without adding a lot of extra
complexity to visual-wrap.el, which I think would have just made it more
prone to mistakes. If you have any other ideas though, let me know.
I also attached the patch I prefer ("match-xdisp.patch"), which is just
what I posted previously with some additional details in the comments.
Would you feel better about this version if I could add regression tests
that ensure xdisp.c does what we expect? In previous work on
visual-wrap, I looked a bit at doing this, and I think it would probably
work if we ran the tests in a GUI frame. That probably requires some
additional scaffolding in the Makefiles, but if it helps improve our
confidence in the display engine, I think it's worth my time to attempt it.
[be-extra-careful.patch (text/plain, attachment)]
[match-xdisp.patch (text/plain, attachment)]
This bug report was last modified 20 days ago.
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