bug#35811: 27.0.50; Arabic character (de)compositions affected by edits elsewhere in buffer

[Eli Zaretskii]

> From: "Basil L. Contovounesios" <contovob@tcd.ie>
> Date: Mon, 20 May 2019 20:03:42 +0100
> 
> 3. C-e RET
> 
> The sheen is correctly shaped in its initial form:
> 
> 3. a ; RET
> 
> The first sheen unexpectedly changes to its isolated form:

I haven't yet found a solution, but I did find a significant clue: in
an Emacs configured with --enable-checking=glyphs, set
inhibit-try-window-id non-nil, and the problem goes away.  So the root
cause is likely in try_window_id or the functions it calls.  Stay
tuned.