Re: Drawing UI elements behind text

[Eli Zaretskii <eliz@gnu.org>]

> Date: Thu, 28 Nov 2024 11:41:25 +0100
> Cc: emacs-devel@gnu.org
> From: Cecilio Pardo <cpardo@imayhem.com>
> 
> On 28/11/2024 10:57, Eli Zaretskii wrote:
> >> 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?
> 
> This is working only if emacs is using a double buffered configuration. 
> Redisplay works on the back buffer, where we don't draw anything. When 
> the back buffer goes to the screen, we add the segments to the screen. 
> scroll_run_hook works on the back buffer.

I see.  So the display code basically knows nothing about these
segments?  And you need to redraw them in their entirety each
redisplay cycle?