Created attachment 339682 [details]
The border color is dimmer for the xterm window

Hi,

I use xterm a lot and I noticed that with mutter the xterm window has a dimmer
border compared to other windows. This happens either in a Wayland or in an
Xorg session.

Is this about Server Side Decorations?

To show the issue more clearly, I am using a partially transparent yellow border:

decoration {
  box-shadow: 0 3px 9px 1px rgba(0, 0, 0, 0.5), 0 0 0 1px rgba(255, 255, 0, 0.5);
}

.ssd decoration {
  box-shadow: 0 0 0 1px rgba(255, 255, 0, 0.5);
}

See the attached image, the Server Side decoration (xterm) looks dimmer
compared to the CSD case (gnome-terminal in a wayland session, but it can be
seen with eog in an Xorg session).

I would expect the border to look always like in the CDS case, this is also
consistent with how Metacity looks in gnaome-flashback (at the bottom in the
attached image).

In https://bugzilla.gnome.org/show_bug.cgi?id=769006#c21 Alberts Muktupāvels
proposes a solution:

[...]
> 4) And for darker shadow - any chance that blend_string used with
> cogl_pipeline_set_layer_combine in meta_shaped_texture.c is not correct?
> Without understanding what I am doing I changed blend_string to "RGB =
> REPLACE (PREVIOUS) A = MODULATE (PREVIOUS, TEXTURE)" (found here [0]) and
> now it looks like shadow is equal between csd and ssd windows.
> 
> [0]
> https://developer.gnome.org/cogl-2.0-experimental/stable/cogl-Blend-Strings.html

Thanks,
   Antonio