Re: Drawing UI elements behind text

[Eli Zaretskii <eliz@gnu.org>]

> Date: Thu, 28 Nov 2024 09:43:21 +0100
> Cc: emacs-devel@gnu.org
> From: Cecilio Pardo <cpardo@imayhem.com>
> 
> On 28/11/2024 7:53, Eli Zaretskii wrote:
> > Those (and other similar ones) are the scenarios where the display
> > engine activates its non-trivial optimization methods, and the
> > questions of interest related to that are:
> > 
> >    . do we need to disable some of the optimizations when these
> >      segments are on display (because their assumptions are
> >      incompatible with how these segments are drawn)?
> >    . how do these segments affect redisplay times in each such case,
> >      after we disable the optimizations, if any, that are incompatible
> >      with them?
> > 
> > These questions are prerequisites for deciding whether we need to make
> > this particular case faster by some special measures.
> 
> The segments are drawn independently of redisplay, on top of its result, 
> without affecting it (so that the next redisplay finds it exactly as it 
> was). So no optimizations are disabled, and the time added by them is 
> independent of what redisplay had to do.

I'm probably missing something: if Emacs uses the scroll_run_hook,
which bitblts a portion of the screen's pixels, doesn't that move also
the pixels of the segments?  If not, does it mean those segments'
pixels are not drawn on the screen, but somehow overlaid on it using
some video driver trick?