update

src/sass/gtk/_common-4.0.scss

@@ -1545,27 +1545,37 @@ searchbar > revealer > box {
 
 headerbar {
   transition: background-color $duration $ease-out, color $duration $ease-out;
-  box-shadow: inset 0 -1px $border;
   background-color: $titlebar;
   color: $titlebar-text;
   min-height: $large-size;
   padding: 0;
   margin: 0;
 
+  @if $rimless == 'false' {
+    box-shadow: inset 0 -1px $border;
+  } @else {
+    border-radius: $window-radius $window-radius 0 0;
+    box-shadow: inset 0 -1px $border, inset 0 1px highlight($titlebar);
+  }
+
   &:disabled { color: $titlebar-text-disabled; }
 
   &:backdrop {
     background-color: $titlebar-backdrop;
     color: $titlebar-text-secondary;
-    box-shadow: inset 0 -1px $border;
 
     &:disabled { color: $titlebar-text-secondary-disabled; }
   }
 
   &.flat, &.flat:backdrop {
     background: none;
-    box-shadow: none;
     transition: none;
+
+    @if $rimless == 'false' {
+      box-shadow: none;
+    } @else {
+      box-shadow: inset 0 1px highlight($titlebar);
+    }
   }
 
   .title {
@@ -1737,10 +1747,15 @@ headerbar {
     min-height: $medium-size;
     padding: 0;
     margin: 0;
-    box-shadow: none;
     border: none;
     outline: none;
 
+    @if $rimless == 'false' {
+      box-shadow: none;
+    } @else {
+      box-shadow: inset 0 1px highlight($titlebar);
+    }
+
     windowcontrols {
       > button {
         @if $window_button == 'mac' {
@@ -1765,10 +1780,20 @@ headerbar {
     }
   }
 
+  .maximized &,
+  .fullscreen &,
+  .tiled &,
+  .tiled-top &,
+  .tiled-right &,
+  .tiled-bottom &,
+  .tiled-left & {
+    box-shadow: inset 0 -1px $border;
+  }
+
   .solid-csd & {
     &:dir(rtl), &:dir(ltr) { // specificity bump
       border-radius: 0;
-      box-shadow: none; // just remove the highlight
+      box-shadow: inset 0 -1px $border; // just remove the highlight
     }
   }
 }
@@ -4984,15 +5009,16 @@ window {
 
   &.csd {
     border-radius: $window-radius;
-    outline-offset: -1px;
-    outline: 1px solid highlight($background);
     margin: 0;
     transition: none;
 
     @if $rimless == 'true' {
+      outline: 0 none transparent;
       box-shadow: $shadow-z16,
       0 0 36px transparent;
     } @else {
+      outline-offset: -1px;
+      outline: 1px solid highlight($background);
       box-shadow: $shadow-z16,
       0 0 36px transparent,
       0 0 0 1px $window-border;

src/sass/libadwaita/widgets/_header-bar.scss

@@ -2,12 +2,18 @@ headerbar {
   min-height: $large_size;
   background-color: $headerbar_backdrop_color;
   color: $headerbar_fg_color;
-  box-shadow: inset 0 -1px $headerbar_border_color;
+  margin: 0;
-  margin: 0 -1px;
   @extend %toolbar;
 
+  @if $rimless == 'false' {
+    box-shadow: inset 0 -1px $headerbar_border_color;
+  } @else {
+    border-radius: $window-radius $window-radius 0 0;
+    box-shadow: inset 0 -1px $headerbar_border_color, inset 0 1px $window_outline_color;
+  }
+
   > windowhandle > box {
-    padding: $base_padding $base_padding + 1px;
+    padding: $base_padding;
 
     > box.start,
     > box.end {
@@ -42,10 +48,15 @@ headerbar {
   &.default-decoration {
     padding: 0;
     margin: 0;
-    box-shadow: none;
     border: none;
     min-height: $medium_size;
 
+    @if $rimless == 'false' {
+      box-shadow: none;
+    } @else {
+      box-shadow: inset 0 1px $window_outline_color;
+    }
+
     > windowhandle > box {
       padding: $base_padding / 2;
     }
@@ -70,6 +81,18 @@ headerbar {
       }
     }
   }
+
+  .maximized &,
+  .fullscreen &,
+  .tiled &,
+  .tiled-top &,
+  .tiled-right &,
+  .tiled-bottom &,
+  .tiled-left &,
+  .solid-csd & {
+    border-radius: 0;
+    box-shadow: inset 0 -1px $headerbar_border_color;
+  }
 }
 
 window > {
@@ -99,12 +122,34 @@ window > {
   }
 }
 
+leaflet:first-child > headerbar {
+  & + separator {
+    background-color: $headerbar_backdrop_color;
+
+    @if $rimless == 'false' {
+      box-shadow: inset 0 -1px $headerbar_border_color;
+    } @else {
+      box-shadow: inset 0 -1px $headerbar_border_color, inset 0 1px $window_outline_color;
+    }
+
+    &:backdrop {
+      background-color: $headerbar_bg_color;
+      transition: background-color $backdrop_transition;
+    }
+  }
+}
+
 %headerbar-flat {
   background: none;
-  box-shadow: none;
   color: inherit;
   min-height: $large_size;
 
+  @if $rimless == 'false' {
+    box-shadow: none;
+  } @else {
+    box-shadow: inset 0 1px $window_outline_color;
+  }
+
   > windowhandle > box {
     padding-bottom: $base_padding;
   }
@@ -116,6 +161,17 @@ window > {
       padding: $base_padding / 2;
     }
   }
+
+  .maximized &,
+  .fullscreen &,
+  .tiled &,
+  .tiled-top &,
+  .tiled-right &,
+  .tiled-bottom &,
+  .tiled-left &,
+  .solid-csd & {
+    box-shadow: none;
+  }
 }
 
 %headerbar-inline {

src/sass/libadwaita/widgets/_popovers.scss

@@ -15,11 +15,18 @@ popover {
     background-color: $popover_bg_color;
     color: $popover_fg_color;
     background-clip: border-box;
+
+    @if $rimless == 'true' {
+      border: none;
+      box-shadow: 0 1px 5px 1px transparentize(black, .91),
+                  0 2px 14px 3px transparentize(black, .95);
+    } @else {
       border: 1px solid gtkmix(white, $popover_bg_color, 8%);
       box-shadow: 0 1px 5px 1px transparentize(black, .91),
                   0 2px 14px 3px transparentize(black, .95),
                   0 0 0 1px $headerbar_darker_shade_color;
     }
+  }
 
   > contents {
     padding: $base_padding;

src/sass/libadwaita/widgets/_sidebars.scss

@@ -8,6 +8,12 @@
     transition: background-color $backdrop_transition;
   }
 
+  @if $rimless == 'true' {
+    headerbar {
+      border-top-right-radius: 0;
+    }
+  }
+
   flap,
   leaflet,
   navigation-view,
@@ -78,6 +84,12 @@
     transition: background-color $backdrop_transition;
   }
 
+  @if $rimless == 'true' {
+    headerbar {
+      border-radius: 0;
+    }
+  }
+
   toolbarview.undershoot-top scrolledwindow {
     @include undershoot(top, $secondary_sidebar_shade_color);
   }
@@ -135,6 +147,12 @@
   }
 }
 
+@if $rimless == 'true' {
+  .content-pane headerbar {
+    border-top-left-radius: 0;
+  }
+}
+
 .sidebar-pane .sidebar-pane {
   background-color: transparent;
   color: inherit;

src/sass/libadwaita/widgets/_toolbars.scss

@@ -244,10 +244,15 @@ toolbarview {
     tabbar    { @extend %tabbar-inline; }
 
     .collapse-spacing {
+      padding-top: 0;
+      padding-bottom: 0;
+
+      headerbar {
+        @extend %headerbar-shrunk;
         padding-top: $base_padding / 2;
         padding-bottom: $base_padding / 2;
+      }
 
-      headerbar { @extend %headerbar-shrunk; }
       searchbar { @extend %searchbar-shrunk; }
       actionbar { @extend %actionbar-shrunk; }
       menubar   { @extend %menubar-shrunk; }

src/sass/libadwaita/widgets/_window.scss

@@ -5,13 +5,14 @@ window {
   &.csd {
     margin: 0;
     border-radius: $window_radius;
-    outline: 1px solid $window_outline_color;
-    outline-offset: -1px;
 
     @if $rimless == 'true' {
+      outline: 0 none transparent;
       box-shadow: $shadow-z16,
       0 0 36px transparent;
     } @else {
+      outline-offset: -1px;
+      outline: 1px solid $window_outline_color;
       box-shadow: $shadow-z16,
       0 0 36px transparent,
       0 0 0 1px $headerbar_darker_shade_color;