[libadwaita/wip/exalm/borderless: 13/19] Revert "Revert "stylesheet: Add back a backdrop state for AdwTabBar""

[Alexander Mikhaylenko <alexm src gnome org>]

commit 43e9287045087ad6faed300f77d3758a1a447f7f
Author: Alexander Mikhaylenko <alexm gnome org>
Date:   Tue Jun 29 13:20:11 2021 +0500

    Revert "Revert "stylesheet: Add back a backdrop state for AdwTabBar""
    
    This reverts commit 97f8aba5dd51ae4254938dd6c3ba5dedd7b862ee.

 src/stylesheet/widgets/_tab-view.scss | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)
---
diff --git a/src/stylesheet/widgets/_tab-view.scss b/src/stylesheet/widgets/_tab-view.scss
index 15d6420a..d238c978 100644
--- a/src/stylesheet/widgets/_tab-view.scss
+++ b/src/stylesheet/widgets/_tab-view.scss
@@ -27,6 +27,12 @@ tabbar {
     border-bottom: 1px solid $alt_borders_color;
   }
 
+  &:backdrop {
+    filter: brightness(.92);
+    transition: $backdrop_transition;
+    transition-property: filter;
+  }
+
   scrolledwindow.pinned {
     undershoot {
       border: 0 solid $alt_borders_color;
@@ -91,6 +97,16 @@ tabbar {
     &:hover {
       background-color: lighten($tab_bg, 3%);
     }
+
+    &:backdrop {
+      // Apply it to children individually so we don't opacify the background
+      stack,
+      .tab-title,
+      .tab-close-button,
+      .tab-indicator {
+        filter: opacity(.5);
+      }
+    }
   }
 
   .start-action,