bug#45054: 27.1; Can get point into the middle of a run of characters with a replacing display spec

[Eli Zaretskii]

> From: Zack Weinberg <zackw@panix.com>
> Date: Sat, 5 Dec 2020 17:12:35 -0500
> 
> type "ambda" in a lisp-interaction-mode buffer
> place point immediately before the first "a"
> M-x prettify-symbols-mode
> type "l"; the word "lambda" will vanish and a λ will appear in its
> place; the visible cursor will be just after the λ
> type "x"; the λ will disappear and "lxambda" will appear, the visible
> cursor will be just after the "x".
> 
> This might mean that the bug also affects writing ordinary text in
> scripts where composite.el is necessary for correct appearance; I'm
> not familiar enough with any of those to say for sure.

The adjustment of point to move it outside of the "forbidden" region
is triggered by point movements, while your examples don't move point.