Re: Auto de-composition when point is inside a composition

[Eli Zaretskii <eliz@gnu.org>]

> From: Stefan Monnier <monnier@iro.umontreal.ca>
> Cc: Kenichi Handa <handa@gnu.org>,  emacs-devel@gnu.org
> Date: Mon, 21 Oct 2019 14:43:32 -0400
> 
> >> - why do we need to set windows_or_buffers_changed at all here?
> >>   after all, this code is only run in some circumstances, definitely not
> >>   all the circumstances where we might need to redraw the grapheme
> >>   cluster after point moved out of it, so it seems it's not the right
> >>   place to handle this case (it seems either redundant or insufficient).
> >
> > Maybe because otherwise the redisplay optimizations taken in case only
> > point moved could fail to redisplay (they might only redraw the
> > character at point, and fail to include the rest of the composition)?
> 
> But in that case, if the command that moved point set
> disable-point-adjustment to non-nil, we'd have a redisplay error, no
> (since we'd then skip this chunk of code).

I think this is about commands that add or remove codepoints from a
potentially composable sequence of characters, and those commands
aren't supposed to play games with this variable.  And we reset the
variable before reading a command anyway.

Btw, I don't have a proof (and doing so would be neigh impossible),
but I suspect we don't need to set windows_or_buffers_changed here,
because it was needed when we had direct insertion and deletion of
characters as part of redisplay.  But given how much surrounding text
HarfBuzzz wants to see to do what its developers consider a good job,
I wouldn't touch this even so.