[libadwaita/wip/exalm/backdrop] stylesheet: Fix shadow transition in dark variant

[Alexander Mikhaylenko <alexm src gnome org>]

commit 19a28a735bfac3fc333b70dcee060c6227c94f98
Author: Alexander Mikhaylenko <alexm gnome org>
Date:   Fri Apr 30 17:16:41 2021 +0500

    stylesheet: Fix shadow transition in dark variant
    
    Add a bogus transparent shadow so that a hairline border doesn't transition
    into a larger shadow.
    
    Fixes https://gitlab.gnome.org/GNOME/libadwaita/-/issues/148

 src/stylesheet/widgets/_window.scss | 1 +
 1 file changed, 1 insertion(+)
---
diff --git a/src/stylesheet/widgets/_window.scss b/src/stylesheet/widgets/_window.scss
index 5c23cb6..e76a53e 100644
--- a/src/stylesheet/widgets/_window.scss
+++ b/src/stylesheet/widgets/_window.scss
@@ -9,6 +9,7 @@ window {
 
   &.csd {
     box-shadow: 0 3px 9px 1px transparentize(black, 0.5),
+                0 2px 6px 2px transparent,
                 0 0 0 1px $_wm_border; //doing borders with box-shadow
     margin: 0px;
     border-radius: $window_radius;