bug#48740: 28.0.50; Composition text property is not always honoured

[Eli Zaretskii]

> From: Ihor Radchenko <yantar92@gmail.com>
> CC: 48740@debbugs.gnu.org
> Date: Mon, 31 May 2021 21:00:29 +0800
> 
> Eli Zaretskii <eliz@gnu.org> writes:
> 
> > No, it is also subdivided into two separate regions, each one of 3
> > characters.  And that's why it doesn't work: the display engine
> > expects that the composed text should be covered by a single text
> > property which specifies the length (7 - 0 = 7) that is equal to the
> > length of the covered text (301 - 294 = 7).
> >
> > So the problem is caused by whatever code in org-agenda-redo-all that
> > "breaks" the composition property into 2 separate adjacent properties.
> 
> Yet, the same "broken" property does work in step 6...

No, AFAIU in step 6 a single property list covers the whole of the
word ONGOING.

> The two intervals are there because other properties in the list are
> not same, as I understand.

Exactly.  And that cannot work with the composition property.

> The only possibility I can think about is that compositions in the two
> intervals are 'eq at step 6, but only 'equal at step 8.

That cannot be the reason, because the display code doesn't test
'eq'uality.  I clearly see in the code that the problem is with
"partial" coverage of the text.