[libadwaita/wip/exalm/backdrop: 17/27] stylesheet: Transparentize header bar contents for backdrop

[Alexander Mikhaylenko <alexm src gnome org>]

commit 863b79cf3554f1ff9f129f51d9185bf139e61453
Author: Alexander Mikhaylenko <alexm gnome org>
Date:   Fri Apr 30 01:29:45 2021 +0500

    stylesheet: Transparentize header bar contents for backdrop
    
    The goal with dropping backdrop styles for GTK4 was to restrict backdrop
    for header bars. However, it's quite complicated and also inconsistent, for
    example leaving window titles unchanged, or removing hover effect from
    regular buttons, but leaving on flat buttons.
    
    Instead, go with a simpler approach, making header bar contents 50%
    transparent. This produces a visually similar result to what it was before,
    but is much simpler.
    
    Unlike the previous method with separate styles, this leaves the hover
    styles unchanged, but still toned down with the rest of the header bar.
    
    Fixes https://gitlab.gnome.org/GNOME/libadwaita/-/issues/107

 src/stylesheet/widgets/_header-bar.scss | 7 +++++++
 1 file changed, 7 insertions(+)
---
diff --git a/src/stylesheet/widgets/_header-bar.scss b/src/stylesheet/widgets/_header-bar.scss
index 97c749e..1206fb8 100644
--- a/src/stylesheet/widgets/_header-bar.scss
+++ b/src/stylesheet/widgets/_header-bar.scss
@@ -22,6 +22,13 @@ headerbar {
     background-image: none;
 
     transition: $backdrop_transition;
+
+    > windowhandle {
+      // opacity looks weird with GtkSwitch, but filter works fine
+      filter: opacity(0.5);
+
+      transition: $backdrop_transition;
+    }
   }
 
   /* Darken switchbuttons for headerbars. issue #1588 */