bug#26742: Display bug with composed strings

[Eli Zaretskii <eliz@gnu.org>]

> From: Clément Pit--Claudel <clement.pitclaudel@live.com>
> Date: Tue, 2 May 2017 02:58:19 -0400
> 
> If I create a file temp.txt containing the following:
> 
> -*- prettify-symbols-alist: (("R_PO" 8477 (Br . cl) 8804)); -*-
> !!! R_PO !!!
> 
> and run ‘emacs-24.5 -Q temp.txt -f prettify-symbols-mode’, I observe a surprising display bug: when I move the point across the second line, as soon as the point reaches the blank space after “R_PO”, I see a second ℝ displayed instead of the third “!”.
> 
> Concretely, the line looks like “!!! ℝ≤ !!!” until I reach the second blank, and then it changes to “!!ℝ ℝ≤ !!!”.  Resizing the window fixes the issue until the next cursor blink.  Additionally, when the point is on R_PO, the point disappears — and taking a screenshot makes the prettified R_PO (ℝ≤) disappear, too (?!).
> 
> Is this a known issue? Can anyone else reproduce it? I have attached a few pictures (all of them show the same buffer text) — let me know if I can provide any other info!

I cannot reproduce this on my system, neither in Emacs 24.5 nor in
25.2 nor in the current master.

If you change the font use for displaying ℝ and ≤, does the problem go
away?