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: Sun, 20 Jun 2021 19:24:55 +0800
> 
> However, I still have no clue what is going on there. I tried to
> investigate further and examined the text properties of the composed
> ONGOING keyword in inbox.org buffer. It turned out that the composition
> is broken in the inbox.org itself:
> 
> #("* ONGOING" 0 1 (...) 2 5 (... composition (0 7 [128119]) 
> prettify-symbols-start 3 prettify-symbols-end 10 face org-todo) 5 9 (... 
> composition (0 7 [128119]) prettify-symbols-start 3 prettify-symbols-end 10 
> face org-todo))
> 
> Yet, the ONGOING string is correctly replaced by 👷 symbol in inbox.org.
> The replacement does not work only in agenda, *and only when concat is
> used to construct the keyword*. I am completely confused about what is
> going on.
> 
> Do you have any ideas about this?

Not sure I understand: ideas about what?  The basic problem is that
display of static compositions requires the values of the composition
property to be 'eq'.  Due to implementation details, when you break a
string with that property into 2 or more parts, the property value(s)
could get copied, in which case they will no longer be 'eq'.  What
else is needed to understand this problem?