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, 30 May 2021 23:13:16 +0800
> 
> At step 8 (after org-agenda-redo-all):
> For "ONG"
> 14 17 (org-heading t org-category \"inbox\" face org-todo 
> prettify-symbols-end 301 prettify-symbols-start 294 composition (0 7 
> [128119]) org-todo-head #(...) fontified t tags (...) org-priority-highest 65 
> org-priority-lowest 67 time-of-day nil duration nil breadcrumbs nil txt 
> #(...) level \" \" time \"\" extra \"\" format (...) dotime t done-face 
> org-agenda-done org-not-done-regexp \"...\" org-todo-regexp \"...\" 
> org-complex-heading-regexp \"...\" mouse-face highlight help-echo \"...\" 
> org-marker #<marker (moves after insertion) at 292 in inbox.org> 
> org-hd-marker #<marker (moves after insertion) at 292 in inbox.org> priority 
> 1001 ts-date nil type \"todo\" todo-state #(...) org-agenda-type todo 
> org-last-args nil org-redo-cmd (...) org-series-cmd nil)
> For "OING"
> 17 21 (org-heading t org-category \"inbox\" face org-todo 
> prettify-symbols-end 301 prettify-symbols-start 294 composition (0 7 
> [128119]) org-todo-head #(...) fontified t tags (...) org-priority-highest 65 
> org-priority-lowest 67 time-of-day nil duration nil breadcrumbs nil txt 
> #(...) level \" \" time \"\" extra \"\" format (...) dotime t done-face 
> org-agenda-done org-not-done-regexp \"...\" org-todo-regexp \"...\" 
> org-complex-heading-regexp \"...\" mouse-face highlight help-echo \"...\" 
> org-marker #<marker (moves after insertion) at 292 in inbox.org> 
> org-hd-marker #<marker (moves after insertion) at 292 in inbox.org> priority 
> 1001 ts-date nil type \"todo\" todo-state #(...) org-agenda-type todo 
> org-last-args nil org-redo-cmd (...) org-series-cmd nil)
> 
> Note the composition part: prettify-symbols-end 301 prettify-symbols-start 
> 294 composition (0 7 [128119])
> The composition part remain unchanged except position in the plist.

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.