[libadwaita/wip/exalm/headerbars: 6/6] stylesheet: Add back a backdrop state for AdwTabBar

[Alexander Mikhaylenko <alexm src gnome org>]

commit 37fc4cb8f15200ad47c3d157450aee16a5493c8f
Author: Alexander Mikhaylenko <alexm gnome org>
Date:   Wed Jun 16 17:17:45 2021 +0500

    stylesheet: Add back a backdrop state for AdwTabBar
    
    Match the new header bar.

 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 616c2685..920d79b8 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,