bug#58793: 28.1.90; Emacs tries to recenter / rescroll when it hits hidden org emphasis

[Ihor Radchenko]

Eli Zaretskii <eliz@gnu.org> writes:

> Btw, you wrote to the Org list that this issue has nothing to do with
> Org, but that is inaccurate.  The bug was triggered by the fact that a
> screen line started with invisible text, and point was inside that
> invisible text.  Emacs display code assumes in a lot of places that
> the position of point is somewhere between the positions of the first
> and the last glyph of a screen line which displays the cursor.
> Invisible text at the beginning or end of a screen line violates that
> assumption, if point is inside invisible text, because invisible text
> doesn't produce glyphs on the screen.  Violating that assumption
> delivers a fatal blow to many redisplay optimizations that reuse
> previous display (instead of redrawing everything anew); one of such
> optimizations is for the situation when nothing's changed on display
> except that point moved.  The display engine is still capable of
> dealing with such unusual situations, but it is likely to recenter the
> window in these cases, something that you find jarring.

Thanks for the explanation!
AFAIU, there is no way to avoid this and also use invisible text
somewhere not at bol/eol. The best we can do is minimizing the chance
to trigger this redisplay behaviour.

> So by using invisible text too much, Org does risk triggering such
> situations more frequently.  Since the result is not fatally wrong,
> just aesthetically annoying, I could say "wontfix", because fixing
> this is notoriously complicated, and risks introducing new exciting
> bugs (I hope I didn't introduce any this time).  In this case I was
> lucky to be able to find a reasonable solution, but I cannot guarantee
> that for any similar issue in the future.  After all, arguably Emacs
> just copes with the unusual situation by falling back on the only
> fail-safe redisplay mechanism we have.  So it is not a bug, strictly
> speaking, just unusual and unexpected behavior.

> Please keep this in mind as you introduce features into Org and
> elsewhere which use invisible text.

Noted. Though I feel that we really need to have A LOT of invisible text
to hit the issue frequently. (here, I was just digging through old bug
reports in Org mailing list; it is not something people often complain
about)

-- 
Ihor Radchenko // yantar92,
Org mode contributor,
Learn more about Org mode at <https://orgmode.org/>.
Support Org development at <https://liberapay.com/org-mode>,
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